"This is a serious hour, a grave hour and the IAEA (International Atomic Energy Agency) must be allowed to conduct its mission to Zaporizhzhia as soon as possible," Rafael Grossi told an emergency meeting of the Security Council via video feed.

Ukraine and Russia have been at war since Moscow invaded its neighbor in late February.

Their forces have clashed at and around Zaporizhzhia, leading to urgent international calls for a cessation of military activity around the plant, the largest of its kind in Europe.

The two sides have traded blame over a recent escalation in fighting around the nuclear facility, which was captured by Russian forces soon after their invasion.

Russia -- a permanent member of the Security Council with veto power -- called for the emergency meeting to address the crisis at the complex.