British Ambassador to the U.S. Peter Mandelson said Sunday that Ukrainian President Volodomyr Zelenskyy must give his “unequivocal” support for President Trump’s push to end the war, insisting there must be a “very radical reset” between the two nations.

Mr. Mandelson also said European nations must ensure an effective military deterrent against further Russian aggression, adding that he hopes the United States could serve as an “ultimate backstop” against Russia “if things get really nasty.”

“It is up to Ukrainians first to come forward,” the British ambassador said on ABC News’ “This Week.” “It is clear that we need to bring the United States and Ukraine back together again.”

Mr. Zelenskyy flew to London to meet with European leaders after Friday’s blowup at the White House.

Mr. Trump chided the Ukrainian leader for warning that Russia poses a threat beyond Ukraine and cast him as ungrateful for U.S. support.

The spectacle raised questions about what would happen if the war continues without U.S. support for Ukraine.

On Sunday, Mr. Mandelson said that Ukraine should commit to a cease-fire to pressure Russia into following suit and that signing a minerals deal with the United States would be wise because it would give the U.S. “a stake in Ukraine’s future.”

Mr. Zelenskyy has signaled support for such an economic agreement but also has said the deal is insufficient because it lacks security guarantees.

“As part of the unfolding plan for this negotiation, the Europeans and perhaps some other countries have to consider how they are going to put forces on the ground to play their part in providing enduring security and deterrence for Ukraine,” Mr. Mandelson said Sunday.

He added the United States also has a role to play.

“We would also like to see the United States giving us cover for this,” the ambassador said. “We want to know the US is covering our backs in what we are putting into Ukraine on the ground and that there is some sort of ultimate backstop.”

“If things get really nasty, the Russians need to know they are going to answer to the United States as well as to the Europeans on the ground,” he said.

• Seth McLaughlin can be reached at smclaughlin@washingtontimes.com.