NATO head Jens Stoltenberg warned Thursday there was still a risk of heavy fighting in Ukraine and urged Russia to halt its support for separatist rebels.

NATO head Jens Stoltenberg warned Thursday there was still a risk of heavy fighting in Ukraine and urged Russia to halt its support for separatist rebels.

A February ceasefire brokered in Minsk by France and Germany has largely held but in recent weeks there has been an upsurge in fighting, threatening to undo the accord as the death toll tops 6,500.

"Ceasefire violations persist. There is still a risk of a return to heavy fighting," Stoltenberg said.

"Russia continues to support the separatists with training, weapons and soldiers; it has a large number of forces stationed on it's border with Ukraine," he added.

"The best chance for peace is the full implementation of the Minsk agreements. I urge all parties to do so and Russia has a special responsibility in this regard."

Russia denies that it is directly involved in the 15-month conflict in Ukraine.

Stoltenberg was speaking at the opening of the NATO-Ukraine Council which the alliance set up to coordinate relations with non-member Kiev after the end of the Cold War.

He said NATO remained committed to Ukraine's territorial integrity and sovereignty, and would not recognize Russia's "illegal and illegitimate annexation of Crimea."