A massive oil slick spotted off Kharg Island has sparked speculation the US has targeted civilian infrastructure on the vital Iranian export hub.
Satellite imagery shows an apparent oil slick as big as the island itself.
But the Pentagon has declined to comment on whether it has targeted the island from which most of Iran's oil is exported.
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With Iran and the US exchanging fire in the Persian Gulf, it is unlikely the spill will be cleaned up any time soon.
Because Iran's coastline is too shallow for massive tankers, 90 per cent of Iran's oil is exported via Kharg Island.
Oil is transported from the mainland to the island for supertankers to take out of the country. The terminal is able to load ten supertankers at once.
A military strike on oil infrastructure there would be devastating to the regime's economy.
It is about 660km away from the Strait of Hormuz.
Meanwhile, the US has struck two Iranian oil tankers.
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In a statement, US Central Command said they had disabled the two vessels as they entered an Iranian port.
The ships were struck in their smokestacks by a F/A-18 Super Hornet.
"US forces in the Middle East remain committed to full enforcement of the blockade of vessels entering or leaving Iran," Admiral Brad Cooper said.
"Our highly trained men and women in uniform are doing incredible work."
Iran's Foreign Minister Abbas Araghchi condemned the attack.
"Every time a diplomatic solution is on the table, the US opts for a reckless military adventure," Araghchi said in a statement.
"Is it a crude pressure tactic? Or the result of a spoiler once again duping POTUS (the President of the United States) into another quagmire?"
Araghchi said in his statement that Iran's missile inventory and launcher capacity was up 20 per cent from the beginning of the war.
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Donald Trump meanwhile has insisted the ceasefire remains in place.
Meanwhile, the United Arab Emirates has reported another missile and drone attack overnight.
The UAE's Defence Ministry said three people were wounded by three drones and two missiles fired from Iran.
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