"We condemn in the strongest terms Russia's continued disregard of its international obligations," Rasmussen said after an emergency NATO meeting on the crisis.
"We urge Russia to cease its illegal military actions, stop its support to armed separatists, and take immediate and verifiable steps towards de-escalation of this grave crisis."
NATO yesterday said Russia had sent at least 1,000 troops to fight with pro-Moscow rebels in Ukraine, along with air defence systems, artillery, tanks and armoured vehicles, and had massed 20,000 troops near the border.