"We have decided, in line with our defensive planning to protect all Allies, to take additional steps to further strengthen deterrence and defence across the Alliance. Our measures are and remain preventive, proportionate and non-escalatory," NATO said in a statement.

A diplomat told AFP that "extra defensive land and air forces are going to be deployed to the alliance's eastern flank, and there will be added naval measures".

The developments were announced after an emergency meeting of ambassadors of NATO's 30 member states hours after Russia carried out air and missile strikes on Ukraine and sent in tanks and soldiers.

NATO chief Jens Stoltenberg was to give a media briefing following the meeting to give details of the decisions.

In its statement, NATO condemned "in the strongest possible terms Russia's horrifying attack on Ukraine, which is entirely unjustified and unprovoked" and a violation of international law and Moscow's commitments.

It said Russia's actions "pose a serious threat to Euro-Atlantic security, and they will have geostrategic consequences".

It also condemned Russian ally Belarus "for enabling this attack".

"Russia's leaders must bear full responsibility for the consequences of their actions. Russia will pay a very heavy economic and political price," it said.

The statement said that the ambassadors consulted under the terms of the alliance's article 4, which is invoked when any party to NATO believes that "the territorial integrity, political independence or security of any of the Parties is threatened".

While Ukraine is not a NATO member, it is a partner country.

The alliance has repeatedly said it will not send combat troops to Ukraine to help defend it from Russia, but some NATO countries have been sending it weapons and ammunition, and deployed extra forces to neighbouring NATO countries.