US Defense Secretary James Mattis has vowed continued American support for Ukraine’s military in the face of Russian "aggression."

Mattis said Thursday that Washington’s sanctions against Moscow would remain in place until it stops supporting pro-independence militias in eastern Ukraine and returns Crimea to Kiev.

"Have no doubt, the United States stands with Ukraine," Mattis said at a press conference in Kiev. with Ukrainian President Petro Poroshenko.

“We support you in the face of threats to sovereignty and territorial integrity, to international law, and to the international order writ large," he said.

Mattis accused Moscow of "seeking to redraw international borders by force.”

"We do not, and we will not, accept Russia's seizure of Crimea and despite Russia's denials, we know they are seeking to redraw international borders by force, undermining the sovereign and free nations of Europe."

Mattis's visit, timed to coincide with Ukraine’s independence anniversary from the former Soviet Union, comes one month after a trip by US Secretary of State Rex Tillerson to Kiev in July.