UN envoy Jean-Marie Guehenno has prolonged his visit to Eritrea in an apparent attempt to meet the government about tensions on the Ethiopian border.
Ethiopia has announced a troop withdrawal from the border
Eritrean President Isaias Afewerki has so far refused to meet Mr Guehenno, the head of UN peacekeeping operations.
Mr Guehenno is trying to reverse an Eritrean order for 180 Western peacekeepers to leave by Friday.
The United Nations Security Council is due to resume negotiations on the subject later on Wednesday.
"[Mr Guehenno] has delayed his departure due to the importance of addressing the current situation," Gail Bindley-Taylor-Sainte, spokeswoman for the UN Mission in Ethiopia and Eritrea (Unmee), told a news conference.
UN flights and a road convoy that were scheduled to evacuate the 180 Western peacekeepers from Eritrea to Ethiopia ahead of a Friday deadline have been put on hold.
Ethiopia and Eritrea fought a costly border war from 1997 to 2000, and implementation of a peace accord has been delayed by Ethiopia's refusal to withdraw from the border town of Badme, which an independent commission awarded to Eritrea.
In recent months Eritrea has placed increasingly tight restrictions on the operations of the UN peacekeeping force deployed on its territory in terms of the peace accord.
Observers say the UN is considering all options, including the withdrawal of the peacekeeping mission, which was established at the end of the Ethiopia - Eritrea border war five years ago.
The BBC's Ed Harris in Asmara says the UN is anxious to keep open the possibility of meeting Eritrean officials and avoid any kind of confrontation.
Eritrea's 10-day ultimatum for North American and European members of Unmee to leave is being seen as an attempt to keep pressure on the international community to persuade Ethiopia to comply with an agreed border demarcation.
Whilst in Ethiopia on Monday, Mr Guehenno warned both countries of the dangers of a renewed border war.
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
A relocation of the affected troops is being considered, the UN says, but the expulsion order is unacceptable as it would set a precedent.
"We cannot accept [the exit order]. If you do, then you change the international character of peacekeeping," Unmee spokeswoman Gail Bindley-Taylor-Sainte told Reuters news agency.
The Horn of Africa neighbours battled over their border in a 1998-2000 conflict that led to some 80,000 deaths.
Under a peace accord, the UN deployed a peacekeeping force in a 25km buffer zone on the Eritrean side of the border and the two states agreed to accept the findings of the border commission.