Russia's defence chief has alleged that Ukraine was preparing a "provocation" involving a radioactive device, a stark claim that was strongly rejected by US, British and Ukrainian officials amid soaring tensions as Moscow struggles to stem Ukrainian advances in the south.

Russian Defence Minister Sergei Shoigu, a close ally of President Vladimir Putin, made the allegations in phone calls with his counterparts from the US, Britain, France and Turkey.

Russia's defence ministry said Shoigu voiced concern about "possible Ukrainian provocations involving a 'dirty bomb,'" a device that uses explosives to scatter radioactive waste. It doesn't have the devastating effect of a nuclear explosion, but could expose broad areas to radioactive contamination.

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Russian authorities repeatedly have made allegations that Ukraine could detonate a dirty bomb in a "false flag" attack and blame it on Moscow. Ukrainian authorities, in turn, have accused the Kremlin of hatching such a plan.

British Defence Secretary Ben Wallace strongly rejected Shoigu's claim and warned Moscow against using it as a pretext for escalation.

The British Ministry of Defence noted that Shoigu, in a call with Wallace, "alleged that Ukraine was planning actions facilitated by Western countries, including the UK, to escalate the conflict in Ukraine."

"The Defence Secretary refuted these claims and cautioned that such allegations should not be used as a pretext for greater escalation," the ministry said.

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The US also rejected Shoigu's "transparently false allegations," White House National Security Council spokesperson Adrienne Watson said in a statement.

"The world would see through any attempt to use this allegation as a pretext for escalation."

In a televised address on Sunday, Ukrainian President Volodymyr Zelenskyy suggested that Moscow itself was setting the stage for deploying a radioactive device on Ukrainian soil.

"If Russia calls and says that Ukraine is allegedly preparing something, it means only one thing: that Russia has already prepared all of it," Zelenskyy said.

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The mention of the dirty bomb threat in Shoigu's calls seemed to indicate the threat of such an attack has risen to an unprecedented level.

The rising tensions come as Russian authorities reported building defensive positions in occupied areas of Ukraine and border regions of Russia, reflecting fears that Ukrainian forces may attack along new sections of the 1000km front line of the war, which entered its ninth month on Monday.

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A Queensland man has made a fortuitous find after spotting a glittering rock that turned out to be a precious gem "the size of a small child's fist".

In video uploaded to TikTok, Matt Betteridge can be heard exclaiming "Holy Dooley" when he releases the large gem from the earth. He declares it to be a "monster" 834 carat sapphire.

Speaking to 9news.com.au, Betteridge said the find left him in absolute shock.

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"We are usually looking for sapphire the size of a fingernail and to find one this size is unreal," he said, revealing the gem has an estimated worth of about $12,500.

"My reaction was disbelief and (we're) absolutely stoked.

"For now we will keep it as a specimen until we decide to sell it – or (get) an offer we can't refuse."

Betteridge uncovered the gem in Rubyvale, near Emerald, which is one of the world's largest sapphire-bearing areas.

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He said the heavy rain that's been bearing down on the east coast over the past few months have made the sapphires easier to find.

Days before he uncovered the 834 carat rock, he made another impressive discovery; a 359 carat sapphire.

At the time that was the biggest sapphire he had found, but little did he know the record would be broken days later.

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Betteridge revealed what prospective prospectors should look out for should they be tempted to try their luck on Queensland's gemfields.

"They just shine like a bit of broken glass," he said.

"Look for your darker stones … they sort of stand out.

"You do get an eye for them eventually."

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A Russian warplane has slammed into a residential building in the Siberian city of Irkutsk, killing both crew members, authorities said.

It was the second time in less than a week that a combat jet crashed in a residential area in Russia.

The Irkutsk region's governor, Igor Kobzev, said the Su-30 fighter jet came down on a private, two-storey building housing two families on Sunday.

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He said that there were no casualties on the ground as the building's five residents were out at the moment of the crash.

He said the residents would be offered temporary accommodation and compensation.

The cause of the crash wasn't immediately known and an official probe has started.

On October 17, an Su-34 bomber crashed near an apartment building in the Sea of Azov port of Yeysk and exploded in a giant fireball, killing 15 and injuring another 19.

The crashes might reflect the growing strain that the fighting in Ukraine has put on the Russian air force.

The United Aircraft Corporation, a state-controlled conglomerate of Russian aircraft-making plants, said in a statement that the plane in Sunday's incident came down during a training flight before its delivery to the air force.

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The jet carried no weapons during the flight.

Surveillance camera videos posted on Russian social networks showed the fighter in a nearly vertical dive and then exploding.

Other videos showed the building engulfed by flames and firefighters deployed to extinguish the blaze.

Irkutsk, a major industrial centre of more than 600,000 in eastern Siberia, is home to an aircraft factory producing Su-30s, a supersonic twin-engine, two-seat fighter that has been a key component of the Russian air force. China, India and many other countries also use the planes.

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Sunday's crash was the 11th reported noncombat crash of a Russian warplane since Moscow sent its troops into Ukraine on February 24.

Military experts have noted that as the number of Russian military flights increased sharply during the fighting, so did the number of crashes.

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A Korean Air Lines plane carrying 173 passengers and crew members has overshot a runway while landing in bad weather in the central Philippines.

All those on board were safe but the airport was closed due to the stalled aircraft, authorities said late on Sunday.

The Civil Aviation Authority of the Philippines said the Airbus A330 from Incheon city in South Korea overshot the runway at Mactan Island in Cebu province prompting the indefinite closure of the airport.

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“All passengers are safe and being attended by ground personnel,” the Philippine aviation agency said in a statement.

At least four incoming flights were diverted to Manila and a nearby airport, it said, but did not immediately give other details, like if the aircraft sustained any damage.

A statement from the South Korea-based airline said the plane attempted to land twice in poor weather, and on the third attempt overran the runway.

It said all passengers made an emergency evacuation via the aircraft’s escape slides as local emergency workers were dispatched to the area.

The airline said it’s trying to find the cause of the incident.

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New South Wales Police are renewing an appeal for information following the mysterious disappearance of a man from Sydney's west last year, which is deemed "suspicious".

Zachary Ritz, 45, was last seen at his unit on Inkerman Street, Granville, on October 27, 2021. He was reported missing almost two months later – on December 21, 2021 – and has not been seen or heard from since.

There are serious concerns for his welfare which has been described as "out of character" and comes after he left all possessions behind.

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Ritz walks with a limp and often uses a walking stick.

He is described of being of Mediterranean/Middle Eastern appearance, of solid build, with shoulder length black curly hair, and a dark scruffy beard.

Cumberland Police Area Commander, Superintendent Andrew Holland, said investigators are particularly eager to speak to anyone who saw or spoke to Zachary in the days leading up to his disappearance.

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"Zachary left all of his possessions and personal belongings behind," Holland said.

"He is an avid fisherman and often travelled to regional parts of the state with mates to fish, so detectives are keen to speak to any of these people who often went on trips with Zachary.

"Strike force detectives will continue to conduct physical searches and other inquiries to determine where Zachary is and the circumstances surrounding his disappearance.

Investigations into Ritz's disappearance continue under Strike Force Alleppo.

Anyone with information about Zachary's disappearance and whereabouts is urged to contact Crime Stoppers