Russian President Vladimir Putin attends a meeting with US President Joe Biden via a video call in the Black Sea resort of Sochi on December 7, 2021. AFP
During their summit by secure video link on Tuesday, Biden warned Putin of a "strong" Western economic response should Moscow attack its neighbour.
"Russia has a peaceful foreign policy, but has the right to defend its security," Putin said at a news conference with Greek Prime Minister Kyriakos Mitsotakis on Wednesday.
He refused to say if he planned to move troops massing on Ukraine's frontier across the border, but said that simply watching NATO move closer to Russia would amount to "criminal inaction."
"We cannot but be concerned about the prospect of Ukraine's possible admission to NATO because this will undoubtedly be followed by the deployment of appropriate military contingents, bases, and weapons that threaten us," he said.
The Russian leader stressed that NATO expanding eastwards is a "very sensitive" issue for Russia.
"It is one of the key questions in preserving Russia's security," he said.
"We have been talking about this all the time, in public, and have been warning our partners that for us (NATO expansion) is unacceptable," Putin said.
Putin told Biden on Tuesday Moscow seeks legal guarantees from the West that Ukraine would not join NATO.
Putin also hailed the talks with Biden as "constructive", saying that the two leaders will continue their dialogue.