United Nations (United States) (AFP) - US Ambassador Nikki Haley on Thursday condemned Russia's "aggressive actions" in Ukraine and pledged strong US support to Kiev even as the new US administration seeks to improve ties with Russia.

In her first public remarks at the Security Council, Haley asserted that US sanctions imposed on Russia over its 2014 annexation of Crimea would remain in place until Moscow gave back the peninsula to Ukraine.

"We do want to better our relations with Russia. However, the dire situation in eastern Ukraine is one that demands clear and strong condemnation of Russian actions," she said.

A surge in fighting in eastern Ukraine has presented the new administration of President Donald Trump with a first test of its Russia policy and Haley's comments were closely watched for signs of any shift.

The US ambassador opened her remarks stating that it was "unfortunate" that her first appearance at the council required her to "condemn the aggressive actions of Russia", describing this as a "replay" of past US statements.

"The United States stands with the people of Ukraine who have suffered for nearly three years under Russian occupation and military intervention," she said.

"Until Russia and the separatists it supports respect Ukraine's sovereignty and territorial integrity, this crisis will continue."

The United States "continues to condemn and call for an immediate end to the Russian occupation of Crimea," she added.

"Crimea is a part of Ukraine. Our Crimea-related sanctions will remain in place until Russia returns control over the peninsula to Ukraine."

While the statements were in line with those of the previous US administration, Russian Ambassador Vitaly Churkin said he detected a change in tone.

"I detected quite a bit of change of tone," he told reporters after the meeting. "It's just the beginning of the road and hopefully the road will lead to something more constructive."

- Ukraine, Russia trade accusations -

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The sharp escalation of fighting since Sunday in the town of Avdiivka has raised concerns of a humanitarian crisis, with some 20,000 people left without heat or water in freezing winter weather.

At least 23 people have been killed in mortar and rocket attacks.

"This escalation of violence must stop," Haley said, pledging US support for the Minsk agreements aimed at ending the conflict.

Ukraine and Russia traded accusations over who was responsible for the flare-up of fighting.

Ukrainian Ambassador Vlodymyr Yelchenko accused Russia of blocking repair work to restore electricity to Avdiivka.

"This is another clear evidence of the Kremlin's intention to create a humanitarian catastrophe in Avdiivka as it was done by the Russian army in Aleppo," he charged, referring to the city seized by Syrian government forces with Russia's military support.

Churkin accused Kiev of igniting fighting to attract international attention and draw support from the new US administration.

"Kiev is trying to use the clashes that they themselves started as a pretext to pull out of the Minsk agreements," said Churkin.

The Minsk agreements, backed by France and Germany, lay out a series of measures to end the conflict in eastern Ukraine -- but their implementation has been faltering.

"Kiev intends in this way to keep on the international agenda the crisis that they themselves started and suck in newly-elected heads of states," Churkin said.

The conflict has killed nearly 10,000 people since 2014, more than 2,000 of whom are civilians.