Speaking at a briefing in Brussels, NATO secretary-general Jens Stoltenberg said that "There is every indication that Russia is continuing to plan for a full-scale attack of Ukraine."

Stoltenberg added that NATO allies currently have over 100 warplanes on high alert and over 120 warships ready at sea, in the area between the Arctic Circle and the Mediterranean. Although NATO's response force has yet to be deployed, he said, some NATO allies are moving troops into the Baltic region.

"NATO has to respond when we see aggressive actions by Russia," he said, "but we continue to strive for dialogue, for de-escalation and we believe it is important to talk."