British Foreign Secretary Liz Truss accused Russia on Saturday of not being "serious about diplomacy" while warning that the "worst-case scenario could happen as early as next week."
Ms Truss also said Ukrainian President Volodymyr Zelensky's speech at the conference was "extremely sobering."
"This is one of the most dangerous moments for European security that we've experienced since early in the 20th century. And we need to show unprecedented unity," Truss said. "There were many people who would want to think hopefully about the situation, but I think we need to prepare for the worst-case scenario, and that worst-case scenario could happen as early as next week."
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Ms Truss said the United Kingdom would stand in unity with its partners to support Ukraine.
"We need to be strong because that is the only thing that Russia understands. And I think we need to be strong in supporting Ukraine and not selling Ukraine out with concessions on sovereignty," Ms Truss said.
"Ukraine needs our support at the United Nations and at the OSCE and they also need our economic support because there is a real threat of economic destabilisation. They need our defence support, and the UK is committed to continuing to supply that support, standing shoulder to shoulder with Ukraine," she added.
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The foreign secretary stressed that the UK is "resolute in imposing severe costs on Russia in the event of an invasion, including tough sanctions."
"We will stop oligarchs being able to move their money internationally. We will stop them traveling, and we will make it tougher for Russian companies to tap into our capital markets. And we're also going to make it harder for Russia to access sovereign debt markets," she said.
Truss said the UK stands with Europe and the United States in being "completely united in support of Ukrainian sovereignty and self-determination."
"This is not just an issue for Europe. This is an issue for the world, because if a sovereign nation is able to be invaded with no consequences, that sends a signal to other aggressors around the world," she added.
"What this crisis has demonstrated is we are united. We are prepared to put tough sanctions in place in the event of an incursion. We are prepared to supply that defensive support to Ukraine," Truss said.
Ukrainian soldiers 'ready for any scenario' as mortar shells explode near front line, interior minister says
A CNN team and other journalists accompanying Ukraine's interior minister on a tour of the front lines in eastern Ukraine came under mortar fire on Saturday.
No one was injured.
Interior Minister Denys Monastyrskiy sought cover as several mortar rounds landed nearby. Shortly after the shelling, he gave interviews to international media in Novoluhanske.
About a dozen mortar rounds landed within a few hundred meters of the group.
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Speaking to CNN prior to leaving the area, Mr Monastyrskiy said, "We spoke with soldiers on the ground. The spirit is incredibly brave and all guys are ready for any scenario."
He said that it had been his first time under fire. He told reporters that he was in the car en route and they had to stop every time they heard shelling and lay on the ground.
At a news conference later in Kramatorsk, Mr Monastyrskiy was asked by CNN what role Ukraine believed that Russian military advisers were playing in the fighting in the eastern part of the country.
"We have information about the advance of the Russian army along our territory," he said. "There is also information that certain units of the Wagner PMC have entered our territory. The purpose of the stay is to organise sabotage in our territory."
