The United Nations peacekeeping head has warned of a "crisis" in the Horn of Africa, as western UN troops left Eritrea after being ordered out.
Eritrea said the peacekeepers must leave by Friday
Almost 90 western troops have left Eritrea. Jean-Marie Guehenno said he had never experienced a similar situation in 10 years in the job.
He has been in Eritrea since Monday in a bid to solve the problems but was not able to meet President Isaias Afewerki.
Ethiopia and Eritrea have both moved troops towards their border recently.
"We are at a critical time. Never has there been such a great crisis for the mission," Mr Guehenno said.
Three plane-loads of UN peacekeepers flew to Ethiopia from Eritrea. They had been told to leave by Friday.
On Wednesday, the UN Security Council decided to evacuate US, Canadian and European staff serving in the mission.
Some 180 personnel are to be moved to neighbouring Ethiopia.
Mr Guehenno said about half of those affected were already on holiday or about to leave anyway, reports the AP news agency.
Most of the UN peacekeepers monitoring the border following a war between the two countries that ended in 2000 are from Asian and African countries and these will stay.
'Sign of frustration'
AFP reports that one of the women on the bus for the airport in Asmara was in tears.
The UN Security Council members said in a unanimously agreed statement, they had decided to move the staff out of Eritrea temporarily in the interest of their safety and security.
Dec 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 members strongly condemned what Eritrea's "unacceptable" actions and restrictions on the peacekeeping mission and warned it could have implications for the operation's future.
Diplomats said the move did not mean they were caving in to Eritrea, which last week ordered the Canadian, American and European staff members to leave the country.
Eritrea's demand is widely seen as a sign of frustration that little has been done by the international community to force Ethiopia to leave the border town of Badme.
This was given to Eritrea by the Boundary Commission, set up under a peace agreement between the two countries.
The Security Council also emphasised the need for progress in implementing this Boundary Commission's decision.
US ambassador to the UN John Bolton said Eritrea was acting unacceptably but that they also needed to focus on Ethiopia's lack of compliance.