US Defense Secretary James Mattis accused Russia of seeking to "redraw international borders by force" and reiterated that Washington "won’t accept" Moscow’s annexation of Ukraine’s Crimea region.
Mattis made the comments in Kiev on August 24, the 26th anniversary of Ukraine’s declaration of independence from the Soviet Union.
"Have no doubt," he said at a joint news conference with President Petro Poroshenko. "The United States stands with Ukraine."
Mattis said sanctions against Moscow would remain in place until it stopped supporting separatists in eastern Ukraine and returns Crimea -- the Black Sea peninsula seized by Russia in March 2014.
He repeated Washington’s commitment to diplomatic efforts to resolve the conflict in Ukraine’s east, where fighting between Kiev’s forces and Russia-backed separatists has killed more than 10,000 people since April 2014.
The defense secretary also accused Russia of not abiding by the February 2015 Minsk agreement meant to put an end to the conflict.
"Despite Russia’s denials, we know they are seeking to redraw international borders by force, undermining the sovereign and free nations of Europe," Mattis said.
He also said that the United States was committed to "building the capacity" of Ukraine’s armed forces.
Mattis added that he was "actively reviewing" the possibility of the United States providing defensive lethal weapons to Ukraine and will inform Secretary of State Rex Tillerson and President Donald Trump about his position on the issue.
Addressing a military parade attended by Mattis and several other Western defense chiefs, Poroshenko earlier said that "Ukraine is ready to give a tough military response to the aggressor if he tries to goes on the offensive" -- a warning to Moscow and the separatists not seek to take more territory in eastern Ukraine.
The United States and the European Union have imposed sanctions on Moscow for its annexation of Ukraine’s Crimea Peninsula and for its support of separatists in eastern Ukraine.