ADDIS ABABA, Jan 21 (Reuters) -- The U.S. embassies in the Ethiopian and Eritrean capitals told non-essential staff on Thursday they could leave as fears mounted that the Horn of Africa nations were on the verge of renewed war.

The failure of recent mediation efforts and a heavy buildup of troops and artillery along the 1,000 km (600 mile) frontier has fuelled speculation of a resumption in fighting over contested border areas after a seven-month lull.

``In response to a continued buildup of troops and equipment along the border... the U.S. government has authorised the departure on a voluntary basis of U.S. embassy personnel in non-emergency positions and family members,'' the U.S. embassy in the Ethiopian capital Addis Ababa said, quoting a statement issued by the State Department.

Last week, the Eritrean government said it had received intelligence reports warning that Ethiopia was about to attack in the next month.

The State Department also called on its citizens to defer travel to Ethiopia and urged U.S. citizens in Ethiopia to review their personal security, the embassy said.

Non-essential staff at the U.S. mission in the Eritrean capital Asmara were also given authorisation to leave.

``It's not based on any specific threat, it's just the possibility of renewed conflict and the build up (of troops) on the border,'' said embassy spokesman Christopher Rowan.

U.S envoy Anthony Lake left the Horn of Africa on Tuesday after his fourth attempt in as many months to end the border dispute which flared into violence last May.

Hundreds of troops were killed in artillery battles on three fronts before fighting subsided with the onset of the rainy season in mid-June.

More than 50 civilians were killed in Eritrean bomb attacks on the northern Ethiopian towns of Mekele and Adigrat in June, while at least one civilian was killed in Ethiopian air raids on Asmara airport, after which Western diplomats and tourists were evacuated from the city.

A peace initiative drawn up in June by the United States and Rwanda failed to resolve the dispute, while later efforts by the Organisation of African Unity based on the same plan have made little headway.