While the military alliance is still not sure what the Kremlin might do, Stoltenberg told an event hosted by the US think-tank the Atlantic Council that Russia had several options available.

"Cyber (warfare) is one, coup efforts to topple the government in Kiev, sabotage -- they have intelligence officers working inside Ukraine as we speak. So we need to be prepared for a wide range of different forms or aggressive actions by Russia against Ukraine," Stoltenberg said.

But he added he was "reluctant to be going too far into speculating, because the aim now is to try to reduce tensions.... We call on Russia to sit down and engage in talks to prevent these kind of scenarios and to find a political solution".

The West is on tenterhooks over a mass deployment of over 100,000 Russian troops on Ukraine's borders, raising fears that Moscow might be planning a further invasion of its neighbour.

NATO has held several meetings on the perceived threat, and has another planned for February 16-17 of allied defence ministers.

Stoltenberg admitted "there are some differences between allies" on what support should be extended to Ukraine, which is not a member of NATO and would not receive any deployment of combat troops.

The US, Britain and Canada are providing weapons and military trainers, while others, such as Germany, are holding back.

"There's no certainty about the Russian plans," he said.

"From the NATO side, we are ready to engage in political dialogue. But we're also ready to respond if Russia chooses an armed conflict, confrontation. So we are ready for both options," he said.

"We are working hard for the best peaceful political solution, but we also prepared for the worst."