The United Nations Security Council has told Eritrea not to expel European and North American peacekeepers from its disputed border with Ethiopia.
Ethiopia and Eritrea fought a bloody war of attrition
It said the order for the troops to leave within 10 days was unacceptable and UN Secretary General Kofi Annan added his voice to the condemnation.
Relations between Ethiopia and Eritrea are poor amid fears of a new war.
The expulsion would make UN observation of the border almost impossible, a BBC correspondent reports from Eritrea.
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.
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.
Reports suggest at least 150 staff may be affected - mainly observers and civilian staff.
Ethiopia has called Eritrea's move "inappropriate and unhelpful".
The two states went to war in 1998. A peace deal in 2000 led to a border ruling by an independent commission.
Mr Annan said in a statement the Eritrean order was inconsistent with the country's obligations to respect the international character of UN staff.
"The United Nations cannot accede to Eritrea's request and demands that the government immediately and unequivocally rescind its decision without preconditions," he said.
The 15 members of the Security Council did not outline any sanctions against Eritrea but said they would consult on how to respond to what they called "the unacceptable action".
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
Eritrea announced its move in a letter which warned the UN mission in Eritrea it was advisable to comply.
The head of UN peacekeeping, Jean-Marie Guehenno, said his impression was that Eritrea was frustrated with the absence of progress on resolving the border dispute.
Eritrea has complained about Ethiopia's refusal to accept the demarcation of their border by an independent commission following the war.
Mr Guehenno said the UN Mission in Ethiopia and Eritrea (Unmee) was already in a difficult situation because of Eritrea's flight ban on UN helicopters and other restrictions but that it was not planning to pull out any of the people who had been mentioned.
"We have no intention of jeopardising a fundamental principle of the universality of the peacekeeping operation representing the whole of the international community," he said in New York.
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.