NATO is concerned over the large number of ceasefire violations in eastern Ukraine, NATO Secretary General Jens Stoltenberg has said.

"We see violations of the ceasefire. And I call on both parties... both parties to respect the ceasefire," he said at a press conference in Brussels.

Stoltenberg said that over the last several months Russia has continued to support separatist militants with heavy weapons, artillery, advanced air defense systems, training, and its own military forces.

"This is also a blatant violation of the ceasefire in the Minsk agreement. Therefore we call on Russia to stop its support for the separatists and to do whatever they can to make sure that the agreement is fully respected," he stressed.

He said that Russia's steady build-up of its military presence along the Ukrainian-Russian border continues.

"There has been a Russian build-up, both along the borders between Russia and Ukraine; but also inside eastern Ukraine....with a steady flow of heavy equipment, tanks, artillery, ammunition, air defense systems and a lot of training. So they have the capacity, the capability to launch new attacks with very little warning time," he said.

Stoltenberg called on Russia to stop its support for the separatists and to do whatever it can to ensure that the Minsk agreement is fully respected.

"We are calling for Russia to withdraw its support for the separatists; because it is in violation of the Minsk agreement; and to stop supporting the separatists and to respect the border between Russia and Ukraine, and to use all its influence on the separatists to make sure that also the separatists fully respect and implement the Minsk agreement," he said.