Mattis said Russia has not abided by the Minsk ceasefire agreement meant to end separatist violence in eastern Ukraine.
“Despite Russia’s denials, we know they are seeking to redraw international borders by force, undermining the sovereign and free nations of Europe,” Mattis told reporters, alongside Ukrainian President Petro Poroshenko.
Mattis’s visit, timed to coincide with Ukrainian Independence Day, is the second high-profile show of U.S. support in as many months, after U.S. Secretary of State Rex Tillerson came to Kiev in July.
Ukraine has counted on U.S. support against Russia since a pro-Western government took power following street protests in 2014 when the Kremlin-backed president fled the country.
But some of President Donald Trump’s comments during the election campaign last year, such as appearing to recognize Crimea as part of Russia, stoked fears in Kiev that Trump might mend ties with Moscow at Ukraine’s expense.
Kiev wants the U.S. to supply lethal defensive weapons to Ukraine — a move that the previous administration under Barack Obama shied away from.
“On the defensive lethal weapons, we are actively reviewing it, I will go back now having seen the current situation and be able to inform the Secretary of State and the President in very specific terms what I recommend for the direction ahead,” Mattis said.