Ethiopia is said to have 100,000 troops on the border with Eritrea
The ICG wants US and UN intervention as the military build up by both countries on the border is at alarming levels.
The International Boundary Commission has given the two countries till the end of November to mark the border physically or it will fix it on maps.
But foreign diplomats fear the deadline could end up being a trigger for war.
Some 80,000 people died during the border war between 1998 and 2000. The disputed town of Badme was awarded to Eritrea in 2002, but Ethiopia insists on talks about the border demarcation.
The United Nations has a peacekeeping force of 1,700 charged with monitoring a security buffer zone on Eritrea's side.
TENSE BORDER 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
Q&A;: Horn's bitter war
"The risk that Ethiopia and Eritrea will resume their war in the next several weeks is very real. A military build up at the common border has reached alarming proportions," an ICG report on the crisis says.
The influential think tank now wants the US and the UN to make it clear to both governments that no destabilising unilateral action will be tolerated.
"There will be no easy military solution if hostilities restart - more likely is a protracted conflict on Eritrean soil, progressive destabilisation of Ethiopia and a dramatic humanitarian crisis," the report said.
The report comes after Eritrea for the third time, accused Ethiopia of planning an invasion.
But Ethiopia has dismissed the claims as a fabrication.
The ICG estimates that Eritrea has 4,000 soldiers supported by artillery and amour in the demilitarised zone and some 120,000 nearby.
And it says Ethiopia maintains 100,000 troops on the border with Eritrea.
The US views Ethiopia as its main ally in the region, while it has named Eritrea as sponsors of terrorism for backing the ousted Somali Islamists.
Nach einem Bericht der International Crisis Group könnte ein Krieg zwischen den beiden Ländern um den umstrittenen Grenzverlauf unmittelbar bevorstehen. Sie verlangt daher ein Eingreifen von UNO und den Vereinigten Staaten in den Konflikt. 

Mittlerweile sind jeweils über 100.000 Soldaten an der Grenze stationiert. Die  International Boundary Commission gab den Kontrahenten bis Ende November Zeit, sich auf eine gemeinsame Grenze zu einigen, oder es werde eine Grenze festgelegt. Ausländische Diplomaten befürchteten, dies könne einen Krieg heraufbeschwören.

Äthiopien hatte am 25. September diesen Jahres damit gedroht, das im Jahr 2000 in der algerischen Hauptstadt vereinbarte Friedensabkommen aufzukündigen. Als Reaktion beschuldigte Eritrea die andere Seite, wiederholt das Abkommen verletzt zu haben.

Etwa 80.000 Tote verursachte die kriegerische Auseinandersetzung zwischen 1998 und 2000. Seitdem sind 1.700 Blauhelme auf dem Gebiet Eritreas stationiert und überwachen die Pufferzone.