Eritrea's decision to expel European and North American peacekeepers from its disputed border with Ethiopia may cripple operations, the UN says.
Ethiopia and Eritrea fought a bloody war of attrition
Tuesday's move will affect 180 staff in areas like supplies, transport and finance, the mission deputy head says.
Earlier the UN's Security Council said the order for troops to leave within 10 days was unacceptable.
The UN is to send two senior peacekeeping officials to Eritrea and Ethiopia to assess the situation.
Under Secretary General for Peacekeeping Jean-Marie Guehenno and military adviser Gen Randhir Kumar Mehta will leave "as soon as practicable", a UN spokesman said.
UN mission deputy chief Joel Adechi said the move was unexpected and the consequences were being reviewed.
A review would establish the detailed impact of the decision on the UN's mission to Ethiopia and Eritrea (Unmee), he told reporters.
"At the end of the assessment we will be able to see if we are able to function like this or what other measures need to be taken."
The mission's capacity to monitor the border had already been reduced more than 60% by restrictions on UN helicopter flights, that were imposed by Eritrea in October.
There has been no explanation for Eritrea's decision to expel the peacekeepers or why personnel from the United States, Canada and Europe including Russia were singled out.
Relations between Ethiopia and Eritrea are poor amid fears of a new war.
But diplomats in Eritrea assume it is an expression of Eritrea's frustration that the international community has done so little to finalise the demarcation of the Ethiopian-Eritrean border, the BBC's Ed Harris reports from Asmara.
Ethiopia has called Eritrea's move "inappropriate and unhelpful".
June 2000: Peace agreement
Apr 2002: Border ruling
Mar 2003: Ethiopian complaint over Badme rejected
Sep 2003: Ethiopia asks for new ruling
Feb 2005: UN concern at military build-up
Oct 2005: Eritrea restricts peacekeepers' activities
Nov 2005: UN sanctions threat if no compliance with 2000 deal
The two states went to war in 1998. A peace deal in 2000 led to a border ruling by an independent commission.
The 15 members of the Security Council have said they will consult on how to respond to what they called "the unacceptable action".
There are some 3,300 peacekeepers and military observers from some 40 countries, 191 civilians and 74 UN volunteers working at Unmee.
UN troops, including some 1,500 from India, patrol a 900km long buffer zone which is just 25km wide and falls on the Eritrean side of the old border.
Ethiopia has not yet withdrawn its forces from the town of Badme, which was awarded to Eritrea.
Frustrated with the stalemate, Eritrea has imposed restrictions on the activities of the UN peacekeeping force patrolling the border buffer zone in the past few months.
Both sides have reinforced their military positions.