Volkswacht Bodensee - King Charles to visit 9/11 memorial in New York

The four-day US tour, clouded by tensions over the Iran war, began in Washington with President Donald Trump warmly greeting the monarch and his wife Queen Camilla.

The New York leg will see the royals meet first responders and victims' families in Manhattan as they mark nearly 25 years since the terror attacks that killed nearly 3,000 people on September 11, 2001.

"This atrocity was a defining moment for America and your pain and shock were felt around the whole world," Charles told the US Congress on Tuesday.

"We stood with you then. And we stand with you now in solemn remembrance of a day that shall never be forgotten," he added in a speech that called for unity among Western powers.

Charles, who is passionate about gardening and the environment, will then visit an urban sustainable farming project providing mentorship to young people and tackling hunger.

Meanwhile, Camilla will celebrate the 100th birthday of fictional teddy bear Winnie the Pooh at the New York Public Library, where she is expected to gift a specially made toy of the character Roo.

Later, Charles will mingle with business leaders, including investors and entrepreneurs, at an event highlighting the transatlantic economic ties.

Trump this month threatened to backtrack on a trade agreement with the UK that limits the impact of US tariffs, as he criticized Britain's lack of support over the Iran war.

Charles's final engagement will celebrate the work of his youth charity, The King's Trust, at a reception that will also spotlight British and American cultural industries.

- 'We stand united' -

Security in New York will be tight for the royal visit, which comes just days after an alleged assassination attempt against Trump at a Washington press gala.

The city's leftist Mayor Zohran Mamdani is not expected to meet Charles privately but will join him for the 9/11 ceremony.

British officials will be pleased so far with the pomp-filled US welcome for Charles and Camilla, which has included a 21-gun salute, military jet flypast and a state banquet at the White House.

Trump, taking a jovial tone, has even joked about his Scottish-born mother having a crush on Charles.

That is in stark contrast to his barbs at Prime Minister Keir Starmer for failing to join the war against Tehran, which had cast a diplomatic shadow ahead of the royal visit.

Charles capitalized on his address to Congress -- the first by a British monarch since his mother Queen Elizabeth II in 1991 -- to paper over those tensions.

"Whatever our differences, whatever disagreements we may have, we stand united in our commitment to uphold democracy," he told lawmakers.

Addressing Congress during celebrations marking 250 years since American independence from Britain, Charles stressed how the two nations' partnership was "born out of dispute, but no less strong for it."

F.Mueller--VB