(CNN) -- NATO has seen a "new Russian military buildup" near the border with Ukraine, NATO chief Anders Fogh Rasmussen said Thursday, a development that may alarm Kiev and the West.

Rasmussen warned that any Russian intervention across the border could prompt tougher international sanctions.

His words, in response to a question from a Russian journalist at an event in London, came a day after Ukraine's new President announced he would soon implement a unilateral cease-fire to ease the crisis in the restive eastern part of the country.

"Yes, I can confirm that we now see a new Russian military buildup -- at least a few thousand more Russian troops deployed to the Ukrainian border, and we see troop maneuvers in the neighborhood of Ukraine," Rasmussen said.