Washington DC - President Donald Trump said Tuesday that the US has no closer friend than Britain, as he welcomed King Charles III to the White House on a state visit shadowed by tensions over the Iran war.

Speaking after a pomp-filled welcome on the South Lawn featuring a 21-gun salute, Trump's tone was a world away from his recent jibes at British Prime Minister Keir Starmer for failing to join in against Tehran.

"In the centuries since we won our independence, Americans have had no closer friends than the British," said Trump, referring to the fact that the royal visit marks the 250th anniversary of the US colonies declaring their freedom from British rule.

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Using a phrase prized by British governments since Winston Churchill first used it after World War II, Trump said that the two countries had a "special relationship, and we hope it will always remain that way."

Trump also praised the British military, saying that "nobody fought better together" with the US – despite recently deriding Britain's two aircraft carriers as "toys."

Afterward, four US jets roared over the White House in a noisy flypast as Trump, Charles, Queen Camilla, and First Lady Melania Trump watched.

On the second day of the four-day royal visit, Charles will later address Congress and is expected to call in his speech for "reconciliation and renewal" amid the recent strains in ties.

Republican Trump has repeatedly lambasted Britain's Starmer over his Iran war opposition, alongside the country's immigration and energy policies.

But Trump – a long-term fan of the British royals who received his own state visit last year – appeared in a jovial mood on a rainy Washington morning as quipped: "What a beautiful British day this is."

The 79-year-old president even joked about how his late mother, who was born in Scotland, "had a crush on Charles," who is now 77.

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Cannons rang out while a military band played the British and US national anthems, before Charles shook hands with top Trump administration officials and inspected troops on the lawn with the president.

A band of soldiers dressed in uniforms from the Revolutionary War then marched past, playing fife and drums.

Trump then held talks with Charles in the Oval Office. But the meeting was held behind closed doors – an unusual step for the usually garrulous US president, reflecting the sensitivity of the visit.

"It was a really good meeting. He's a fantastic person. They're incredible people, and it's a real honor," Trump told reporters afterward.

In the evening, the royal couple will return to the White House for a grand state dinner.

Security has been tight for the trip, which comes just days after an alleged assassination attempt against Trump at a Washington press gala. Countersniper teams could be seen on the roof of the White House.

Melania Trump told reporters she was "very well" when asked how she was doing after the shooting.

The first day of the trip featured a more informal welcome, with the Trumps treating Charles and Camilla to tea and cakes before showing them the White House beehives.

But the second day will feature Charles' most public-facing moment, when he becomes the first British monarch to address Congress since his mother, the late Queen Elizabeth, in 1991.

With tensions still high over Iran, Charles is expected to appeal to Trump in guarded terms, saying that defending common democratic ideals is "crucial for liberty and equality."

"Time and again, our two countries have always found ways to come together," he is expected to say.

But in an unwelcome distraction, Britain's new ambassador to Washington, Christian Turner, was quoted by the Financial Times as telling students during a recent visit that America's special relationship is "probably Israel."

His predecessor, Peter Mandelson, was sacked over links to sex offender Jeffrey Epstein. King Charles is under pressure to acknowledge Epstein survivors during the visit as his brother Andrew was forced to give up his title of prince over the scandal.