UN Secretary-General Kofi Annan says he is very concerned about reports that Ethiopia and Eritrea have moved troops closer to their common border.
Mr Annan called on both sides to exercise maximum restraint.
Last month, Eritrea banned UN peacekeeping helicopters from its air space and restricted troop movements.
UN peacekeepers patrol the border under the terms of a 2000 peace agreement that followed a two-year border war between Eritrea and Ethiopia.
Mr Annan called on both sides to immediately stop any action that could be misinterpreted or could jeopardise security arrangements agreed in their peace plan.
Since Eritrea banned UN helicopter flights, the activities of UN peacekeepers have been severely constrained, and they have been forced to abandon many of their posts.