The United Nations has accused Eritrea of moving 1,500 troops into the buffer zone that separates the country from its neighbor Ethiopia.
The United Nations called the incursion Monday a "major breach" of a six-year ceasefire between the two countries.
U.N. officials say the Eritrean troops, backed by 14 tanks, also took over a U.N. mission checkpoint in the buffer zone.
U.N. Secretary General Kofi Annan urged Eritrea to withdraw its troops immediately, saying the move could seriously jeopardize the peace process.
There was no immediate comment from Eritrean authorities.
Eritrea has long complained that Ethiopia is violating a 2000 peace agreement by refusing to accept a border delineation set out by an international panel.
Eritrea and Ethiopia fought a two-year border war from 1998 to 2000 that killed an estimated 70,000 people.