The US Secretary of State has insisted there would be no easing of sanctions against Russia until Moscow reversed its actions in Ukraine.
In a pledge of support for Kiev, Rex Tillerson said that the US would continue to hold Moscow to the Minsk Agreement, the deal that starts with a ceasefire and the withdrawal of heavy weapons from the front lines in the war-torn east of the country.
At a meeting of NATO in Brussels on Friday (31 March), Tillerson said: "American and NATO support for Ukraine remains steadfast.
"As we have repeated at every ministerial and summit since Russia launched its campaign of aggression against Ukraine, NATO allies stand firm in our support of Ukraine's sovereignty and territorial integrity.
"We do not, and will not, accept Russian efforts to change the borders of territory of Ukraine," he added.
The comments are different in tone to those made by President Donald Trump who has been reluctant to criticise Russia over the seizure of Crimea.
Earlier on Friday, the US Defense Secretary James Mattis criticised the Kremlin for "mucking around" in other country's elections.
The vote of support by the US comes at a critical time for Ukraine, whose President Petro Poroshenko ordered the military to implement a cease-fire in the Donbass region starting on 1 April.
The Ukrainian military has said that a colonel was killed in the city of Mariupol and that eight of its soldiers had been killed this week alone, Agence France Presse reported.
Rainer-Elk Anders is a Ukraine analyst who has just come back from the war-torn Donbass region. There he interviewed Ukrainian soldiers in towns including Donetsk, Marinka and Adiivka, which was the scene of shelling in January that cut off its water supply.
He told IBTimes UK that although they lack logistical support and equipment, soldiers are digging their heels, spurred on by the sense that it would be difficult for separatist forces to take large swathes of Ukrainian territory.
"If we look at the changing map of the frontline of the last four or five months, the Ukrainian army has made very small, strategic, territorial gains.
"The picture at the moment is that in certain parts, there is very heavy war activity, but there are parts where the Ukrainian army has made progress, small strips where the frontline has shifted in favour of the Ukrainians," he said.
"The soldiers have a feeling they are fighting for Ukraine – they are not fighting for the government," he added.