The North Atlantic Treaty Organisation (NATO) on Thursday called for de-escalation of Russian military build-up on the eastern borders of Ukraine, vowing to protect the territorial integrity of Ukraine while offering the opportunity for negotiation with Russia.

"We call on Russia to return to diplomacy, to de-escalate, and to respect Ukraine's sovereignty and territorial integrity," said NATO's Secretary General Jens Stoltenberg after meeting with Ukrainian President Volodymyr Zelenskyy in Brussels.

Stoltenberg warned that "any further aggression against Ukraine will have severe consequences and would carry a high price." Zelenskyy said he favored a peaceful solution. "We do believe that the diplomatic format negotiation will win."

The call was echoed by some EU leaders gathering on Thursday for their last summit this year. Xavier Bettel, Prime Minister of Luxembourg, called for open discussion between all involved parties. He said that the EU should extend an invitation to Moscow to try and appease tensions through talks.

EU heads of state are also discussing and preparing potential sanctions against Russia, should the country choose not to reduce its military presence on the Ukrainian border.

"Any aggressive action will meet with a higher political and economic cost," said Josep Borrell, High Representative of the EU for Foreign Affairs and Security Policy, on Thursday.

He said the bloc is preparing "a full set of sanctions." Russian President Vladimir Putin has underlined the need to immediately begin negotiations on clear and internationally legal agreements against NATO's further eastward expansion and against the deployment of weapons in Russia's neighbors, primarily Ukraine.