NATO blamed Russian-backed separatists on Monday for a sharp escalation of violence in eastern Ukraine and called on Moscow to stop destabilizing Ukraine.
Alliance Secretary-General Jens Stoltenberg criticized Russia's actions and supported Ukraine's independence after NATO and Ukrainian ambassadors held an extraordinary meeting in Brussels.
Kiev had asked for the meeting in response to a new rebel offensive in eastern Ukraine.
"We condemn the sharp escalation of violence along the ceasefire line in eastern Ukraine by Russia-backed separatists," Stoltenberg told a news conference after the meeting.
Stoltenberg dismissed as "nonsense" allegations by Russian President Vladimir Putin that a NATO legion was fighting alongside Ukraine government troops in the east of the country.
"There is no NATO legion. The foreign forces in Ukraine are Russian," he said.
Despite repeated commitments to work for a peaceful solution, Russia continued to provide the separatists with support, training, forces and advanced equipment, including rocket systems, heavy artillery, tanks, armored vehicles and electronic warfare systems, he said.
"We call on Russia to stop its support for the separatists immediately, to stop destabilizing Ukraine and to respect its international commitments," Stoltenberg said. Russia denies being the driving force behind the rebellion.
NATO has made clear it has no plans to intervene militarily in Ukraine, which is not a NATO member, but it has stepped up cooperation with Kiev, including help with modernizing its armed forces.
Diplomats said Ukraine was mainly looking for political support from NATO at the meeting rather than any concrete aid.
Faced with the worsening situation on the ground, Ukraine has asked some individual countries for additional military assistance, Ukraine's Ambassador to NATO Ihor Dolhov said, but he did not name them or say if any weapons were being supplied.