The United Nations says Eritrea has placed new restrictions on UN peacekeeping operations along the border with Ethiopia.
Badme remains under Ethiopian occupation
It says with Eritrea limiting some night patrols the possibility of a military build-up cannot be ruled out.
Earlier this month the UN criticised an Eritrean ban on UN helicopter flights.
UN troops patrol the border zone under the terms of the 2000 peace plan that followed a two-year border war in which 70,000 people died.
"In some places we have received sporadic orders that after sunset at 1800 (1500 GMT), the vehicles are not allowed to move," UN commander Colonel Mohammed Aqbal told reporters, AFP reports.
"Overall, our monitoring capabilities have been reduced by 55%," he said.
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.
UN Secretary General Kofi Annan emphasised that both governments had the primary responsibility to bring about peace and security.
Earlier this year, the UN expressed concern over a build-up of Ethiopian and Eritrean troops close to the border zone.
Eritrea became independent from Ethiopia in 1993, after Ethiopian and Eritrean rebel movements overthrew the Derg regime in Addis Ababa.