An aerial photo shows the British Indian Ocean Territory (BIOT), or Chagos Islands, in the Indian Ocean, halfway between Africa and Indonesia. The territory comprises a group of more than 60 individual islands, the largest being Diego Garcia, the site of a joint military facility of the U.K. and the U.S. Pictures From History/Universal Images Group/Getty

U.S. Secretary of State Marco Rubio and his British counterpart were to meet Friday in Washington amid tension between the allies over the future of the remote Chagos Islands, and specifically over the potential use of Britain's Diego Garcia military base on those islands in any new U.S. attack on Iran.

President Trump slammed British Prime Minister Keir Starmer in a social media post this week over a deal reached last year for the U.K. to hand the Chagos Islands over to Mauritius, but to maintain control of Diego Garcia via a 100-year lease.

Mr. Trump called it "a big mistake" and urged Starmer not to "give away Diego Garcia," warning in a post on his Truth Social platform that "should Iran decide not to make a Deal" with the U.S. on its nuclear program it "may be necessary for the United States to use Diego Garcia, and the Airfield located in Fairford [England], in order to eradicate a potential attack by a highly unstable and dangerous Regime."

On Tuesday, the U.S. State Department issued a statement backing London's agreement with Mauritius for the tiny nation to take sovereignty over the British Indian Ocean territory. But Mr. Trump decried it the following day in his social media post as "an act of GREAT STUPIDITY," making the link to his threat to strike Iran if the Islamic Republic doesn't agree to a new deal to rein in its nuclear program.

The Times of London, along with other British news outlets, reported Thursday that the U.K. government had denied the U.S. permission to use both Diego Garcia and the Royal Air Force's Fairford base in England — both of which host American long-range bombers — for new strikes on Iran.

The U.K. Ministry of Defense declined to comment directly on operational matters, but a government spokesperson told CBS News on Thursday that Britain supported the ongoing political process between the U.S. and Iran, adding: "Iran must never be able to develop a nuclear weapon, and our priority is security in the region."

CBS News confirmed that the issue of Diego Garcia would be discussed in Washington on Friday in the meeting between Rubio and U.K. Foreign Secretary Yvette Cooper.

President Trump has ordered a huge U.S. military buildup around Iran, including both warships and air power, as he pressures Tehran to make a deal on its nuclear enrichment program.

The deployments include the USS Abraham Lincoln aircraft carrier strike group, already in the Arabian Sea, and a second strike group, led by the biggest carrier in the world, the USS Gerald R. Ford, which was seen crossing the Strait of Gibraltar Friday on its way into the Mediterranean. Those deployments will put both strike groups, including their associated warships and dozens of aircraft, within striking distance of Iran.

A senior regional official told CBS News on Thursday that one reason the U.S. is moving two aircraft carriers to the region is the reluctance of some U.S. allies to grant permission for their territory to be used in any strikes on Iran. Saudi Arabia, Jordan, and UAE have all said their airspace and territory cannot be used for strikes by any party.

Iran, meanwhile, has protested at the United Nations over Mr. Trump's threat of a possible attack if the ongoing diplomacy fails.

In a letter sent Thursday by Iran's U.N. mission to the members of the Security Council, obtained by CBS News, the Islamic Republic called Mr. Trump's "belligerent statement" about the potential use of Diego Garcia in an attack a "flagrant violation" of the U.N. charter and international law that it said risked "plunging the region into a new cycle of crisis and instability."

The letter said Iran had "engaged constructively, with seriousness and in good faith, in nuclear talks" with the U.S., and it called on the U.N. and its members to "act without delay, before it is too late" to "ensure that the United States immediately ceases its unlawful threats of the use of force."

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