"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 defence 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 armoured vehicles across the border from Russia to Ukraine."
Stoltenberg urged Moscow to "use all its influence on the separatists to make them respect the ceasefire".
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"We will provide practical support for Ukraine, with modernising and reforming the defence army and of course we are also adapting our own defence 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 defences with six command centres in eastern Europe and a spearhead force of 5,000 troops, to counter what the alliance called Russian aggression in Ukraine.
The six "command and control" centres 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.
All six countries were once in the Soviet Union's orbit and have voiced deep concern about Russia's actions in Ukraine.