The United Nations may have to consider withdrawing its monitors from the tense Ethiopia-Eritrea border, Secretary General Kofi Annan has warned.
A UN force has been patrolling the border for several years
He said the Eritrean government had not been cooperating with the UN and had limited the movement of the UN troops.
The UN border mission announced the immediate withdrawal of peacekeepers from 18 of its 40 monitoring posts.
Eritrea banned helicopter flights two weeks ago. The two-year war between Eritrea and Ethiopia ended in 2000.
Since then, more than 3,000 UN troops have been monitoring the disputed 1,000km border.
"Obviously, our procedures and operations have been impeded," Mr Annan told reporters.
"If this continues, we will have to take some very hard and critical decisions as to the usefulness of staying there if we cannot operate."
About 70,000 people are believed to have died in the conflict.
The two countries ended their border war after a peace agreement which provided for an independent commission to rule on their disputed border.
However, the commission's findings have yet to be implemented, and Ethiopia continues to occupy the town of Badme, which the commission ruled belonged to Eritrea.
Last month, Eritrea warned it might re-start its war with Ethiopia if the UN failed to resolve a border dispute between the two neighbours.
Eritrea became independent from Ethiopia in 1993, after Ethiopian and Eritrean rebel movements overthrew the Derg regime in Addis Ababa.