"Every indication is that Russia continues to plan for a full-scale attack on Ukraine," Stoltenberg said after an urgent meeting with Ukraine's envoy.

"We see that more and more of the forces are moving out of the camps and are in combat formations and ready to strike."

Stoltenberg said "further Russian troops" had already moved across the Ukrainian border overnight into the Kremlin-backed territories.

"What we see is further invasion of a country which was already invaded," Stoltenberg said.

Moscow seized the Crimea peninsula peninsula from Ukraine in 2014 and has fuelled a separatist conflict in eastern areas for eight years that has killed over 14,000 people.

Russian President Vladimir Putin attempted Monday to again rewrite his pro-Western neighbour's borders by recognising the breakaway regions as independent states.

Putin has warned he could now send his forces across into the rebel territories depending on how the situation develops.

The recognition has drawn condemnation from the West. Europe and the US are posied to impose sanctions on Moscow.

NATO allies spearheaded by the US have already sent thousands of additional troops to bolster their eastern flank as tensions have soared with Moscow.

Stoltenberg said NATO put it its response force on higher readiness several weeks ago and has 100 jets on high alert and 120 ships at sea.