Secretary-General Ban Ki-moon urged Ethiopia and Eritrea on Wednesday “to exercise maximum restraint” following clashes on their disputed border.

Eritrea accused Ethiopia of “military aggression” by attacking its positions in the Tsorona Central Front, a border area that saw one of the bloodiest battles during their 1998-2000 border war. Ethiopia’s military said its troops were provoked into launching a counter-offensive after Eritrean forces fired into Ethiopian positions on Sunday.

The U.N. chief met Ethiopian Prime Minister Hailemariam Desalegn in Brussels on Wednesday and Deputy Secretary-General Jan Eliasson phoned Eritrean Foreign Minister Osman Saleh Mohammed.

“They urged both Governments to exercise maximum restraint and refrain from any act or statement that could exacerbate the situation,” U.N. spokesman Stephane Dujarric said.

“They also called on both governments to resolve their differences through peaceful means, including by ensuring the full implementation of the peace agreement they signed in 2000,” he said.

Dujarric said the U.N. is available to assist in any peace efforts.

Eritrea and Ethiopia have been feuding over their border since Eritrea gained independence from the Addis Ababa government in 1993 after a 30-year guerrilla war.

A 1,700-strong U.N. force monitored a 15-mile (24-kilometer) wide, 620-mile (1,000-kilometer) long buffer zone between the Horn of Africa neighbors under the December 2000 peace agreement.

But tensions between the two countries remain high because of Ethiopia’s refusal to accept a boundary commission’s 2002 ruling on the border demarcation which awarded the key town of Badme to Eritrea. The Eritrean government progressively limited peacekeepers’ movements in response and it July 2008 the Security Council ended the peacekeeping mission.

Secretary-General Ban warned that that a new war could break out if U.N. peacekeepers withdraw.

Dujarric said the U.N. now has no way to monitor “what is actually going on along the border.”