Poroshenko arrives in Washington seeking more robust US military assistance to help his country in its fight against Russian-backed rebels. Obama so far has resisted Ukraine's request for lethal assistance, though the US has provided about $60 million in nonlethal aid to Ukraine's military.

White House officials made clear that Poroshenko's visit — his first to the US since being elected this summer — was aimed in part at sending a message to Russia about the West's backing for the embattled former Soviet republic.

Ukraine and Kremlin-backed separatists have been locked in a months-long fight for control of eastern Ukrainian cities that sit on Russia's border, aggression that followed Russia's annexation of the strategically important Crimean Peninsula. The US and Western allies have condemned Russia's provocations, levying a series of economic sanctions and restricting President Vladimir Putin's involvement in some international organizations.

In recent weeks, the West has accused Russia of moving troops and equipment across its border with Ukraine, though the Kremlin denies such involvement.

Ukraine and the Russian-backed separatists inked a cease-fire agreement Sept. 5, though the deal has been violated repeatedly. On Wednesday, shelling in rebel-held parts of the east killed at least 12 civilians, as a top leader of pro-Russian rebels rejected Ukrainian legislation meant to end the unrest by granting self-rule to large swathes of the east.