Britain's defence ministry said the US had begun using the military sites for "specific defensive operations to prevent Iran firing missiles into the region".

Later Saturday, US President Donald Trump rejected what he said were British plans to possibly send two aircraft carriers to the region -- hours after having mocked Prime Minister Keir Starmer for his initial reluctance to get involved in the conflict.

Starmer annoyed Trump for initially refusing to have any role in the US-Israeli war with Iran, which began on February 28.

He later agreed to a US request to use two British military bases for a "specific and limited defensive purpose".

Those bases are Fairford in Gloucestershire, southwestern England, and the UK-US Diego Garcia base on the Chagos Islands in the Indian Ocean.

A US Air Force B-1 Lancer bomber landed at Fairford on Saturday, an AFP photographer saw.

An American C-5 Galaxy plane could also be seen on the runway of the base, as anti-war protesters demonstrated outside.

- Trump's broadside -

Earlier in the week, Trump had said he was "not happy with the UK" and mocked Starmer, saying "this is not Winston Churchill that we're dealing with".

Then in a post late Saturday on his Truth Social platform he dismissed what he said were British plans to possibly send two aircraft carriers to the region.

"The United Kingdom, our once Great Ally, maybe the Greatest of them all, is finally giving serious thought to sending two aircraft carriers to the Middle East," he posted.

"That's OK, Prime Minister Starmer, we don't need them any longer -- But we will remember," he said. "We don't need people that join Wars after we've already won!"

Earlier Saturday, the BBC and other UK media speculated that the HMS Prince of Wales, one of Britain's two aircraft carriers, could be deployed to the Mediterranean, though the reports said no decision had yet been taken.

A defence ministry spokesperson said: "HMS Prince of Wales has always been on very high readiness and we are increasing the preparedness of the carrier, reducing the time it would take to set sail for any deployment."

Starmer, a former human rights lawyer, has defended his initial decision by saying any British action "must always have a lawful basis and a viable, thought-through plan".

He has also insisted he was right to change that position because Iran's retaliation with missiles and drones to the US-Israeli strikes had threatened British interests and allies in the region.

Lawmakers in Starmer's ruling Labour party remain haunted by former prime minister Tony Blair's disastrous support for the US-led invasion of Iraq in 2003.

A Survation poll of 1,045 Britons published Friday found that 56 percent of respondents believed Starmer was right not to involve Britain in the initial strikes. Only 27 percent said he was wrong.

Several thousand people, many waving Iranian flags, marched through central London to the US embassy on Saturday to protest against the war.

Some demonstrators waved placards with slogans including "Stop Trump's Wars" and "Stop Arming Israel".

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