Prince Harry has lobbed a grenade into the delicate relationship between the US and Britain with a high-profile speech urging President Donald Trump to do more to help end the war on Ukraine.

The Duke of Sussex also directly appealed to Russian President Vladimir Putin to “stop this war”, as he condemned the “systematic war crimes” of the Ukrainian conflict.

Harry delivered a lengthy, impassioned speech during a surprise visit to the war-torn nation on Thursday (local time). Earlier, he was filmed by ITV News arriving at Kyiv railway station, stepping off an overnight train from Poland and greeting and hugging people on the platform.

Later, in his speech at the Kyiv Security Forum, he said he was “not here as a politician” but as “a soldier who understands service”, and as a humanitarian.

In a rare input on global issues, the King’s youngest son also turned his attention to the US – although he did not mention Trump by name – urging the “American leadership” to “honour its international treaty obligations” in its “enduring role in global security”.

“The United States has a singular role in this story,” he said. “Not only because of its power, but because when Ukraine gave up nuclear weapons, America was part of the assurance that Ukraine’s sovereignty and borders would be respected.”

“This is a moment for American leadership. A moment for America to show that it can honour its international treaty obligations, not out of charity, but out of its own enduring role in global security and strategic stability.”

Harry’s Ukraine speech comes at a tricky time for the King, who is due to head to the US. Photos: AAP

The speech may become a thorny issue for Harry’s father, who begins a four-day state visit to the US on Monday amid a transatlantic relationship already strained by the war and other tensions.

The royal visit will kick off with a private tea with Trump before the King addresses the US Congress. There will also be a state dinner and visits to New York and Virginia.

On Friday, Trump said the King and Queen’s visit could “absolutely” mend relations with Britain. He hailed the monarch as a “great man” and said the royal couple’s trip would be “a positive”.

“He’s fantastic. He’s a fantastic man … I know him well, I’ve known him for years,” he said.

“He’s a brave man, and he’s a great man. They would absolutely be a positive.”

But he also warned he didn’t think British Prime Minister Keir Starmer had any chance of repairing the relationship unless he changed direction.

The two leaders have been at loggerheads over the conflict in the Middle East, with Trump repeatedly deriding Starmer as weak and indecisive.

Asked in the Oval Office later on Thursday about Harry’s comments, Trump initially deflected, referring briefly to Meghan Markle – who he has previously criticised.

“Prince Harry? How’s he doing? How’s his wife?” Trump said. “Please give her my regards.”

He went on to add: “I know one thing: Prince Harry is not speaking for the UK. That’s for sure. I think I’m speaking for the UK more than Prince Harry. But I appreciate his advice.”

Next week’s royal state visit marks the 250th anniversary of ‌the US declaration of independence from British rule, when the then 13 American colonies decided to split from King George III, Charles’s five-times-great-grandfather.

For the King, it will be a time to reflect on how Britain and the US have ‌united since then to forge some of the world’s closest security, military and economic ties while for Trump it will be another chance to indulge his love of the royals.

Nigel Sheinwald – Britain’s ambassador to the US from 2007 to 2012 – said the visit could not, and was not designed to, heal any ​current acrimony between governments. But it would demonstrate ties that went far deeper than any individuals.

“Pretty much more than any other visit, this is about the long term. This is about the fundamentals of the relationship between our peoples, our countries,” Sheinwald told Reuters.

“It’s not about what’s going on today.”

Royal aides privately say Trump behaved impeccably during his unprecedented two state visits to Britain in 2019 and last year.

“He is a huge royalist,” ​royal biographer Robert Hardman told Reuters.

“He has … one mindset when it comes to the British government but British monarchy is a completely separate element, and he’s a huge fan of it. And he adored the late queen, a big fan of the King. For him, this is a big moment.”

-with AAP