WASHINGTON — Embattled US President Donald Trump faced explosive allegations that he divulged top secret intelligence to Russian diplomats in the Oval Office, a charge the White House scrambled to rebut on Monday.

The Washington Post reported that Trump revealed highly classified information on the Islamic State group during a meeting last week with Russian Foreign Minister Sergei Lavrov and Moscow’s man in Washington Sergey Kislyak.

In a shock twist, the intelligence reportedly came from a US ally who did not authorise Washington to share it with Moscow. That development could shatter trust that is essential to intelligence and counterterrorism co-operation.

National Security Advisor HR McMaster denied the president had revealed "intelligence sources or methods," but acknowledged that Trump and Lavrov "reviewed a range of common threats to our two countries, including threats to civil aviation."

The Post, citing unnamed officials, said that Trump went off script during the meeting, describing details about an Islamic State terror threat related to the use of laptop computers on airplanes, revealing the city where the information was gathered.

The Trump administration recently barred the use of laptops in the passenger cabin from several countries in the Middle East and is mulling the expansion of that ban to cover jets originating in Europe.

"There’s nothing that the president takes more seriously than the security of the American people. The story that came out tonight as reported is false,"

McMaster said without elaborating on which elements were wrong.

"Two other senior officials who were present, including the secretary of state, remember the meeting the same way and have said so. Their on-the-record accounts should outweigh those of anonymous sources. I was in the room. It didn’t happen."

McMaster earlier refused to answer questions to a group of journalists gathered in the West Wing, saying "this is the last place I wanted to be" before leaving.

The revelations are the latest in a wave of crises to hit the White House, which late Monday was in a state of shock, with aides frantically trying to put out the fire and determine the source of such damaging leaks.

Since coming to office in January, Trump has lurched from crisis to crisis, lampooning the intelligence services, law enforcement and the media along the way.

Last week, Trump threw his administration into turmoil by taking the virtually unprecedented step of firing his FBI director James Comey.

Comey had been overseeing investigations into possible Trump campaign collusion with Russia to skew the 2016 election.

The meeting came a day after that firing, and was already controversial in itself, a red carpet welcome for top aides of Vladimir Putin just months after being hit with US sanctions for meddling in the 2016 presidential election.

Trump’s administration was left red-faced after Moscow surprised them by releasing pictures of what was meant to be a closed-door meeting.

Ryan wants ’full explanation’

For Trump’s already weary allies in Congress, the latest crisis brought more headaches and demanded yet more explanation from an administration that is struggling to leave its legislative mark.

"We have no way to know what was said, but protecting our nation’s secrets is paramount," said Doug Andres, a spokesman for Republican House Speaker Paul Ryan.

"The speaker hopes for a full explanation of the facts from the administration."

Senior Republican Senator John McCain told CNN that "if it’s true, it’s obviously disturbing." But he cautioned: "Let’s wait and see what this was all about first."

Top Senate Democrat Chuck Schumer accused Trump of potentially putting American lives at risk.

"If the report is true, it is very disturbing. Revealing classified information at this level is extremely dangerous and puts at risk the lives of Americans and those who gather intelligence for our country," he said.

"The president owes the intelligence community, the American people and Congress a full explanation." — AFP