(CNN) US Defense Secretary James Mattis vowed continued US support for Ukraine in the face of Russian "aggression" as he met with the country's leaders in Kiev on Thursday.

Mattis attended a Ukranian Independence Day parade in the capital before sitting down with his Ukrainian counterpart, Defense Minister Stepan Poltorak, and President Petro Poroshenko.

Speaking alongside Poroshenko at a news conference, Mattis said he intended to strengthen the US relationship with Ukraine in the face of Russia's annexation of Crimea in 2014 and the ongoing conflict with pro-Russian separatists in eastern Ukraine.

"Have no doubt, the United States stands with Ukraine. 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.

"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."

US Defense Secretary James Mattis (left) and Ukrainian President Petro Poroshenko meet in Kiev, Ukraine, on Thursday.

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