"Any further provocation or aggressive actions by Russia would be of serious concern. We call on Russia to be transparent about its military activities," Stoltenberg said after a meeting with Ukraine's foreign minister.

Germany on Monday also urged Russia to exercise "restraint" at the Ukrainian border amid growing concern in the West about Russian troop movements in the area.

"In order not to further aggravate an already tense situation, it's important for Russia to exercise restraint with regard to military activities and movements" along the border with Ukraine, German foreign ministry spokesman Christofer Burger told reporters. "We are watching these military activities by Russia with concern."

*This story was edited by Ahram Online