Biden's visit is his fourth to Kiev since Russia annexed Ukraine's Black Sea peninsula in March 2014 and then watched with approval as pro-Kremlin insurgents carved out their own region in the eastern industrial heart of the ex-Soviet state.
The war-scarred nation now fears slipping off the global radar due to recent international efforts to enlist Russia in a joint fight against the Islamic State (IS) group. But Biden brought a message of reassurance. "The US stands firmly with the people of Ukraine in the face of continued, I emphasise, continued aggression from Russia and Russian-backed separatists," he told Ukrainian President Poroshenko.
Washington's EU allies support Ukraine's view of Russia orchestrating and backing the separatist revolt in reprisal for the February 2014 ouster of a Moscow-backed president - an assertion the Kremlin denies. Both Washington and Brussels have slapped stiff economic sanctions on Russian President Vladimir Putin's inner circle and have helped train and equip Ukraine's underfunded army with defensive equipment such as advanced radar.
But the situation changed when Russia began launching ferocious air strikes against Syrian President Bashar al-Assad's foes on September 30. Washington has accused Putin of trying to prop up Assad by targeting Syria's western-backed rebels instead of extremist groups.
Yet the Islamic State's claim of downing a Russian airliner carrying 224 people over Egypt on October 31 appears to have prompted Moscow to focus more on bombing IS's oil infrastructure and other jihadist targets. The November 13 Paris attacks which were also claimed by IS saw France pushing to enlist Russia in a "grand coalition" against the jihadist group alongside the United States and some European and Arab states.
Biden said the latest developments in the Middle East in no way appeased Washington's anger with Putin and support for Poroshenko's pro-Western team.
"The invasion by Russia of Crimea will not be accepted by us or by the international community. This attempted annexation is contrary to international law, is wrong, and will not be accepted in any circumstance," he said.
Biden also spoke out on the issue of graft in a country of 40 million where morale is sagging due to Poroshenko's seeming inability to tackle the corruption that has plagued Ukraine for much of its recent history.
Poroshenko's prosecutor general has been accused of blocking investigations and hiring workers who have since been detained with huge stashes of gold and cash in their flats. "It's absolutely critical for Ukraine... to root out the cancer of corruption," the US vice president said.