Russia and pro-Moscow rebels in eastern Ukraine now have the capacity to launch new attacks "with very little warning" after a sustained military build-up, NATO head Jens Stoltenberg said Monday.

"In eastern Ukraine, Russia and the separatists (now) have capabilities which enable them to launch new attacks with very little warning time", violating a ceasefire agreed in February, Stoltenberg said.

US Defense Secretary Ashton Carter warned last week that the rebels were clearly "preparing for another round of military action," a view shared by senior US military officers who have cited  Russia's increased activity as cause for concern.

"There has been a Russian build-up on the Ukraine border and also inside eastern Ukraine with a steady flow of heavy equipment, tanks and air-defence systems," Stoltenberg told reporters at NATO HQ in Brussels.

"No one can say anything about their intentions but we are calling on Russia to stop supporting the separatists, to respect the border and use all its influence with the separatists to fully respect the Minsk agreement," he added.

France and Germany, together with Russia, brokered a second ceasefire between Kiev and the rebels in the Belarus capital Minsk in February after a September agreement collapsed.

The accord called on both sides to withdraw heavy weapons, allow full access to OSCE monitors to police the ceasefire and restore border control to Ukraine.

While fighting has died down, both sides accuse each other of violations on a daily basis and the OSCE says it is being prevented from doing its job.

Stoltenberg said the Minsk accord was the only way forward and all sides had to respect its provisions if there was to be a peaceful solution.

The conflict, sparked in April 2014 as Ukraine cemented ties with the European Union and after Russia annexed the Crimea peninsula, has so far claimed more than 6,100 lives and plunged Western ties with Moscow into the deep freeze.

In what might be a sign of a slight thaw, US Secretary of State John Kerry is meet Russian President Vladimir Putin in the Black Sea resort of Sochi on Tuesday.