Donald Trump failed to commit to helping the UK in a potential Falkland Islands conflict - saying it was "not there to help me" in the Iran war.
Last week, reports suggested that the US may oppose British sovereignty over the territory if the government does not boost funding for defence.
The Pentagon is reportedly reviewing allies' commitment to NATO and devising measures to incentivise meeting the 5% defence spending pledge.
Argentine President Javier Milei is due to make a national televised statement concerning the Falklands late on Thursday.
Asked if he would "come to the aid" of the UK over the Falklands, the US president told GB News: "Well look, I was there when they had the first war. That was a long time ago and I watched it so carefully, and you comported yourselves very well.
"You took it back pretty strongly, but it's a long way away. That's a big trip, it's a long way.
"So, your country's not doing well. Don't forget, you get a big percentage of your oil from the Strait of Hormuz, and you weren't there to help me.
"Your country was not there to help me. We didn't need help, but I did ask NATO, and I asked your former leader, 'why didn't you send a couple of ships?' He said we don't have any, and it was pretty sad, the whole thing."
An internal Pentagon email was leaked in April, suggesting that the US may review its position on the islands.
The US provided support to Britain after the Argentinian invasion in 1982, though it maintains a neutral position.
The government last week defended the UK's contributions to NATO and reaffirmed its position on the Falklands.
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A spokesperson said: "The UK position on the Falkland Islands is long-standing and unwavering: they are and will remain a British Overseas Territory in line with the wishes of the Falkland islanders.
"The UK is playing a central role in NATO and strengthening collective defence across Europe.
"We are NATO's third largest cash spender on defence and the only European NATO member to commit our nuclear deterrent to the alliance."
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