President Joe Biden told eastern European allies known as the Bucharest Nine that "The commitment of the United States to NATO ... is absolutely clear. Article 5 is a sacred commitment the United States has made."
"We will defend literally every inch of NATO, every inch of NATO," Biden added.
Article 5 stipulates that an attack on any one member of NATO is treated as an attack on all, requiring a joint response.
At the meeting, Biden reaffirmed Washington's commitment to their security.
"As NATO's eastern flank, you are the front line of our collective defence. You know better than anyone what is at stake in this conflict. Not just for Ukraine, but for the freedom of democracies throughout Europe and around the world."
"There should be no doubt: our support for Ukraine will not waver, NATO will not be divided, and we will not tire," he said.
Biden is conferring with leaders of countries on NATO's eastern flank which have led calls for military assistance to Ukraine and where there is widespread concern that the conflict could spill over.
Biden, on a visit to Poland on Tuesday, said Ukraine would "never be a victory for Russia", while Vladimir Putin vowed to press on with the nearly year-long war.
Meanwhile, as Biden has been building engagement with NATO allies and Ukraine, Moscow has been seeking support from Beijing.
Chinese Ambassador to Russia Wang Yi's visit to the Kremlin comes after Washington and NATO voiced concern that China could be preparing to supply Russia with weapons to pursue its war in Ukraine.
Beijing has sought to position itself as a neutral party, while maintaining close ties with strategic ally Russia.
It has said it is "deeply concerned" and that the conflict is "intensifying and even getting out of control".
China has promised to publish a proposed "political solution" to the Ukraine conflict this week.
At the start of the meeting, Putin said cooperation between Russia and China was "very important to stabilize the international situation."
