NATO Secretary General Anders Fogh Rasmussen on Sunday showed concerns over the tension in Eastern Ukraine, urging Russia to de-escalate the crisis.

"I am extremely concerned about the further escalation of tension in Eastern Ukraine. We see a concerted campaign of violence by pro-Russian separatists, aiming to destabilize Ukraine as a sovereign state," Rasmussen said in a statement.

"I call on Russia to de-escalate the crisis and pull back its large number of troops, including special forces, from the area around Ukraine's border," he added.

"If Russia is serious about its desire to de-escalate the crisis in Ukraine, it should stop blaming others for its own actions, pull back its troops, step back into line with its international obligations and start rebuilding trust," Rasmussen also argued in an article entitled "De-escalation starts on the ground" published on Sunday.

Russia's response to the remarks is not yet available.