"It is still a fluid situation. What we have seen is that the Ukrainian forces are fighting bravely and are actually able to inflict damage on the invading Russian forces," Stoltenberg said after a video summit of NATO leaders.

"It is a full invasion of Ukraine. They are moving towards Kyiv and the stated goal is to change the government of Ukraine."

Stoltenberg's warned that the Kremlin's aggression had created a "new normal", threatening Europe's broader security beyond non-NATO member Ukraine.

"We have already strengthened our deterrence and defence," Stoltenberg said.

"Yesterday, allies activated our defence plans and, as a result, we are deploying elements of the NATO Response Force (NRF) on land, at sea, and in the air to further strengthen our posture and to respond quickly to any contingency."

The step is the latest by NATO aimed at beefing up its defences after allies spearheaded by the United States rushed thousands of troops to eastern members as the Kremlin moved on Ukraine.

"We have over 100 jets at high alert, operating in over 30 different locations and over 120 ships from the high north to the Mediterranean," Stoltenberg said.

"This is to preserve peace to prevent an attack and to prevent that the war which is going on in Ukraine spills over to any NATO allied country."

He did not give any details on where the response forces were being sent, saying it was up to NATO's top military commander.

Created in 2003, the NRF is made up of 40,000 personnel and includes an 8,000-strong high-readiness contingent with air, sea and special operations soldiers that can be deployed within days.

In Washington, Pentagon spokesman John Kirby said the United States had troops currently in Europe, plus 7,000 ordered to deploy to German this week, and others on standby at home.

He said which troops and how many depend on the specific needs of NATO.

"Whatever those requirements are... the United States is ready to lean forward as much as possible," Kirby said.

He stressed the NRF was to protect NATO countries, not to engage in fighting in Ukraine, which is not a member of the alliance.

However, he said, the trigger for activating the NRF "has been this unlawful invasion by Russia into Ukraine."

"We're going to do what we need to do to defend every inch of NATO territory." he added.