The United Nations has condemned Eritrea, accusing it of preventing hundreds of peacekeepers from crossing from Eritrea into Ethiopia.
The UN has held an emergency session over its stranded force
The UN ordered its regional force to withdraw to Ethiopia after the Eritrean government cut off its fuel supplies.
But the UN says only six vehicles have been allowed to leave, some troops have been threatened at gunpoint and now their rations have been stopped.
Eritrea denied blocking their departure saying its supplies had simply run out.
In an emergency session on Friday, the 15-member UN Security Council expressed "deep concern about the impediments and logistical constraints" faced by the force.
Jean-Marie Guehenno, head of the UN Department of Peacekeeping Operations, told reporters: "It's a very serious situation. We're running out of fuel, we're running out of food."
The UN peacekeepers were sent eight years ago to monitor a border security zone after the war which killed tens of thousands of people between Eritrea and Ethiopia in the late 1990s.
Dec 2000: Peace agreement
Apr 2002: Border ruling
Mar 2003: Ethiopian complaint over Badme rejected
Oct 2005: Eritrea restricts peacekeepers' activities
Nov 2005: UN sanctions threat if no compliance with 2000 deal
Dec 2007: Deadline to demarcate the border expires without agreement
Dec 2007: Eritrea cuts fuel supplies to the UN
Feb 2008: UN warns of a return to war as it withdraws from Eritrea
The Eritreans are angry about Ethiopia's failure to hand over the disputed town of Badme - which a boundary commission set up after the war awarded to Eritrea - and want the international community to put pressure on Ethiopia to withdraw.
Having lost patience with the UN, Eritrea cut off fuel supplies to the UN's 1,400 troops and 200 military observers.
The BBC's Elizabeth Blunt in Ethiopia says Eritrea may be hoping the UN force will abandon some vehicles and communication equipment.
But the Eritrean ambassador to the UN, Araya Desta, told the BBC his country simply did not have any more supplies to spare.
He added that the Security Council's reaction had been "outrageous".
"To victimise the victim is ridiculous and shameful for the council," Mr Desta said.
The government in Addis Ababa said it would co-operate in letting the troops relocate to northern Ethiopia.