Biden, insisted that food aid and money would be sent to save Ukrainian lives, and that refugees from the conflict would be welcome in America.
He said: "We will defend every single inch of Nato territory with the full might of a united and galvanised Nato.”
But he added: “We will stand together with allies in Europe and send an unmistakable message.
We will not fight a war against Russia in Ukraine. Direct confrontation between Nato and Russia is World War Three, something we must strive to prevent."
He said Nato was "one movement", which is why he had moved thousands of US troops along the borders with Russia.
"Because they move once, granted, if we respond, it is World War Three," Mr Biden said.
"But we have a sacred obligation on Nato territory, a sacred obligation, Article Five ... although we will not fight the Third World War in Ukraine. Putin’s war against Ukraine must never be a victory.
“The idea that we're going to send in offensive equipment, and have planes and tanks and trains going in with American pilots and American crews - just understand. Don't kid yourself. No matter what you all say, that's called World War III. Okay? Let's get it straight here guys."