NATO leaders will agree to strengthen forces on the eastern flank of the alliance at the summit on Thursday by deploying four new battle groups in Bulgaria, Hungary, Romania and Slovakia, NATO Secretary-General Jens Stoltenberg said, Reuters reported.

"I expect leaders will agree to strengthen NATO's posture in all domains, with major increases in the eastern part of the alliance on land, in the air and at sea," Jens Stoltenberg told a news conference ahead of the summit in Brussels.

With the new battle groups and existing forces in the Baltic states and Poland, he said, NATO would have eight multinational formations along the alliance's eastern flank. Stoltenberg said the Ukrainian crisis showed that NATO needed to reconsider its deterrence and defense posture for the long term.