President Donald Trump agreed to meet alliance leaders in Europe in May in a phone call on Sunday with NATO Secretary General Jens Stoltenberg that also touched on the separatist conflict in eastern Ukraine, the White House said.

Trump was elected on a pledge to push NATO members to increase their funding to the western alliance to ease the financial burden on the United States.

This proposal has drawn opposition from both his fellow Republicans as well as Democrats and the idea has worried European allies who fear Russian President Vladimir Putin might take advantage.

A White House statement said Trump and Stoltenberg "discussed how to encourage all NATO allies to meet their defense spending commitments."

"President Trump agreed to join in a meeting of NATO leaders in Europe in May," the statement said.

Trump and Stoltenberg also "discussed the potential for a peaceful resolution of the conflict along the Ukrainian border."

Over the past week a flare-up in hostilities has erupted between the Ukrainian military and Russia-backed separatists, with each accusing the other of a new wave of shelling. More than 40 people have been killed in both government- and rebel-held areas.

Trump has drawn fire at home for wanting to warm up ties with Putin.

In an interview broadcast on Sunday during Fox Channel's Super Bowl pre-game show, Trump waved off concern from interviewer Bill O'Reilly that "Putin's a killer."

"We’ve got a lot of killers...You think our country’s so innocent? You think our country’s so innocent?" Trump said, citing the 2003 war in Iraq.

U.S. Senator Marco Rubio of Florida, who had lost to Trump in the Republican presidential primary battle last year, tweeted that it would be a mistake to lift U.S. sanctions on Russia, a step Trump has been considering.

"Only reason we should ever lift sanctions on #Putin is if he meets conditions of sanctions & ends violations of #ukraine sovereignty," he said.

Senate Majority Leader Mitch McConnell, a Kentucky Republican, told CNN's "State of the Union" he was not going to critique everything Trump says but on Russia, "I obviously don't see this issue the same way he does."