All 28 NATO allies fully support Ukraine as it faces the worst upsurge in fighting against pro-Russian rebels in two years, alliance deputy head Rose Gottemoeller said Thursday.

US President Donald Trump has stoked concerns in NATO and Europe by dubbing the alliance "obsolete" and taking a softer stance on Russia, in marked contrast to his predecessor.

"There was unanimity around the NATO-Ukraine Council table, strong support from all allies for Ukraine," she told reporters after talks with Ukrainian Prime Minister Volodymyr Groysman at NATO headquarters in Brussels.

The US-led alliance had stood by Ukraine since "Russian aggressive actions" began in 2014 and it would not recognise the illegal annexation of Crimea early that year, she said.

The recent fighting was the worst in two years, with OSCE monitors reporting more than 10,000 violations of the Minsk ceasefire accords on one day, she said.

"We are deeply concerned by the recent spike in violence... We must not accept this as the new normal," she said, urging all parties and especially Russia to honour their commitments to the Minsk deal.

Trump told Ukrainian President Petro Poroshenko last week in a telephone call that he would work with both Kiev and Moscow to end the conflict, but fears persist he might prefer to deal directly with Russian leader Vladimir Putin.

Gottemoeller, a former US under-secretary of state who was nominated for the NATO post last year by then-president Barack Obama, stressed NATO's full support.

Groysman said the whole world was aware of "Russian aggression in eastern Ukraine... we value highly the unanimous support of all the allies."

Asked about possible policy changes under Trump, Groysman said he had no concerns.

"I am sure that the new president... will always fight for democracy and democratic values," he said.

"I believe the United States will always support justice and justice is on the side of Ukraine."