Former British Prime Minister Boris Johnson has met with Republican lawmakers, pressing the United States to sustain aid to Ukraine to help it fight off Russia's assault.

Johnson spoke with US House of Representatives Speaker Kevin McCarthy, the top Republican, in his office and is scheduled to speak at a private Republican club in the evening, said Representative Joe Wilson, a member of the House Foreign Affairs Committee.

Johnson is also scheduled to meet with a group of Republican senators, said US Senator Todd Young, though he said he was unaware of the exact timing.

On Wednesday, Johnson will discuss the need for "Western unity and support for Ukraine and what more can be done against the threat Russia poses" at the Atlantic Council think tank.

The Republicans took over the House from the Democrats at the start of this year and some hardline members among the Republicans have called for an end to US military and other assistance to Ukraine, which amounts to tens of billions of dollars.

"I am here primarily to recognise and pay tribute to the immense US contribution to the security of the Ukrainian people," Johnson said in a statement.

"My mission is to demonstrate that Ukraine will win – and that there is no conceivable case for delay in further supporting the Ukrainians to win this year.

"We should have no fear of escalation when it comes to the provision of weaponry."

Johnson, who left office in September following a series of scandals, was prime minister when Russia invaded Ukraine in February last year. He sought to position the UK as Ukraine's top ally in the West and has continued to do so.

During a trip to Ukraine this month, Johnson visited Borodyanka and Bucha, suburbs of the capital Kyiv that became a byword in the West for Russian atrocities. Moscow denies allegations by Ukraine and its Western allies that it has committed war crimes.

In a Washington Post opinion piece published on Monday, Johnson argued for the admission of Ukraine to the NATO security alliance.

“Ukrainians should be given everything they need to finish this war, as quickly as possible, and we should begin the process of admitting Ukraine to NATO, and begin it now,” he wrote.

Wilson, who will be among the lawmakers meeting Johnson, is a staunch Ukraine supporter, having even suggested the placement of a bust of Ukrainian President Volodymyr Zelensky in the US Capitol.

Johnson’s office did not respond to a request for comment, nor did the office of House Speaker Kevin McCarthy, the top Republican lawmaker.