An internationally-agreed ceasefire in Ukraine is being violated by Russia "again and again", NATO's chief said on Wednesday.

NATO Secretary-General Jens Stoltenberg also criticised new Russian snap military checks on combat readiness, saying they undermined stability.

"The ceasefire is violated again and again, and this is of great concern," Stoltenberg told a news conference following a meeting about Ukraine with NATO defence ministers. "Russia supports the separatists ... with equipment, with weapons. They also mass troops along the Ukrainian border," he said.

European Union leaders must decide this month whether to extend sanctions on Russia over the crisis.