Rasmussen estimated the buildup was of "at least a few thousand more troops," and called it "a very regrettable step backwards."
His comments confirmed warnings made by Ukrainian officials in the past several days, noting the increasing militarization of the border. On Thursday, Ukraine's National Security and Defense Council claimed, in a statement, new troops and equipment were in place immediately over the border. It added a military convoy -- nearly 10 miles long -- was observed on a road between Moscow and Kiev, the capital of Ukraine.
The statement noted four Russian units were identified on the border: two airborne divisions, an airborne assault brigade and a motorized rifle brigade.
Sporadic fighting in the Donetsk region of eastern Ukraine continued after pro-Russian separatists rejected a ceasefire and call to disarm, proposed by Ukrainian Petro Poroshenko. Ukraine is expected to honor its own proposal, unilaterally respecting a ceasefire, in several days.
The ceasefire is the first step in a 14-point peace proposal by Ukraine that includes sealing its border with Russia, disarming and offering amnesties to pro-Russian insurgents, and providing safe passage to those willing to leave the country.