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

In recent weeks there has been an upsurge in fighting in a conflict which has already killed some 6,500 people, despite a February ceasefire brokered in Minsk which has been regularly breached.

"Ceasefire violations persist. There is still a risk of a return to heavy fighting," Stoltenberg said on the second day of a NATO defence ministers' meeting at its Brussels headquarters.

"Russia continues to support the separatists with training, weapons and soldiers; it has a large number of forces stationed on its 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."

Stoltenberg rejected suggestions by reporters that the ceasefire violations and continued Russian support for the rebels effectively meant the Minsk peace deal was dead.

"I can see no alternative but to continue to support peace efforts... without Minsk, I am really afraid the situation could deteriorate."

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

NATO's 28 members have approved a series of measures to boost its readiness and reassure nervous allies in eastern Europe that it will stand by them in the face of what Stoltenberg said Wednesday were Russia's "aggressive actions in Europe".

He said the NATO-Ukraine Council, which the alliance set up to coordinate relations with the non-member state after the end of the Cold War, had reviewed efforts to support Kiev in a series of political, economic and military reforms.

He added that the fight against corruption was especially important to ensure Ukraine could stand on its own two feet.

NATO also agreed to help Ukraine manage air traffic, with neighbouring states such as Poland supplying information to improve oversight and safety.

Stoltenbergy made no direct reference to the July shooting down of Malaysia Airlines MH17 over rebel-held territory in the east but said it was prudent to improve air traffic control in an area where there was fighting on the ground.

NATO remained committed to Ukraine's territorial integrity and sovereignty, and would not recognise Russia's "illegal and illegitimate annexation of Crimea," he added.