"The safety of our personnel is paramount, so staff have been relocated to Lviv and Brussels. The NATO offices in Ukraine remain operational," the official told AFP, without giving numbers.

Several Western countries have already moved diplomats from Kiev to Lviv, located near the border with Poland, in anticipation of Russian military action.

Brussels hosts NATO's headquarters.

The NATO official said: "NATO and allied countries are monitoring and assessing the situation very closely, and continue to take all the necessary measures."

The United States dominates NATO, and US President Joe Biden on Friday said he was "convinced" Russia was going to invade Ukraine within the week, and have its forces target Kyiv.