A large number of troops being deployed at the border between Ethiopia and Eritrea is worrying the United Nations.
A UN spokeswoman said the troops remained on their own soil, but Ethiopian soldiers had come within a 20-40 kilometre range of the frontier.
A UN force has been patrolling the border for several years
This could threaten the stability of the temporary security zone and the UN urged both countries to show restraint.
Ethiopia and Eritrea fought a war
between 1998 and 2000 that is thought to have killed more than 70,000 people.
"We are monitoring the situation very closely but at the moment the situation remains stable," said Gail Bindley-Taylor Saint for the for the UN Mission in Ethiopia and Eritrea (UNMEE).
The European Union has already said it is worried by the military build-up along the 1,000km border.
A Western diplomat said on Monday on condition of anonymity that Ethiopia had an estimated 60,000 troops near the border - almost double the number there late last year.
The diplomat said the bulk of Eritrea's 300,000 force was also near the buffer zone.
The UN mission has also confirmed that it is investigating Ethiopian allegations that it captured five suspected Eritrean soldiers and killed another two during a clash inside Ethiopia.
The peace process stalled in 2002, when Ethiopia rejected an international boundary commission ruling, demarcating their border.
This had awarded Badme, the symbolic town where the war started, to Eritrea.
Although Ethiopia said last year it had abandoned its opposition to the commission's ruling, the two countries have yet to agree on how to implement its findings.
Both sides have been invited to a meeting with the boundary commission later this month.