RIGA, Latvia — NATO on Wednesday urged Russia to withdraw all forces from eastern Ukraine and to end its support for the pro-Moscow separatists as a ceasefire appeared to unravel.

"I urge Russia to withdraw all its forces from eastern Ukraine, to stop all its support for the separatists and to respect the Minsk agreement," NATO Secretary General Jens Stoltenberg said in the Latvian capital Riga.

"The ceasefire has not been respected," he told reporters before meeting with EU defence ministers and said he was "deeply concerned by the deteriorating situation" in eastern Ukraine.

"Russian forces, artillery and air defense units as well as command and control elements are still active in Ukraine," he said, adding that "there has been a steady buildup of tanks and armored vehicles across the border from Russia to Ukraine."

"The refusal of the separatists to respect the ceasefire threatens the agreement as does the denial of access to the area for OSCE monitors."

Stoltenberg urged Moscow to "use all its influence on the separatists to make them respect the ceasefire."

"We will provide practical support for Ukraine, with modernizing and reforming the defense army and of course we are also adapting our own defense posture to the fact that we see that the security environment in Europe is changing because of the actions of Russia in Ukraine."

NATO agreed earlier this month to dramatically boost its defenses with six command centers in eastern Europe and a spearhead force of 5,000 troops, to counter what the alliance called Russian aggression in Ukraine.

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Britain, France, Germany, Italy, Poland and Spain have agreed to take the lead in forming the spearhead rapid reaction force, which would be available to deploy within a week in a crisis.

The six "command and control" centers that will help the deployment of the force will be in Bulgaria, Estonia, Latvia, Lithuania, Poland and Romania, with a corps headquarters in Szczecin, Poland.