The United Nations security council expressed concern over the clashes and urged both sides to exercise restraint.
'Eritrean aggression'But Alejandro Wolff, the US deputy permanent representative to the United Nations, focused most of his condemnation on Asmara.
"The council needs to speak up about Eritrea's aggression against Djibouti," he said.
"There is a pattern of irresponsible, destabilising behaviour by Eritrea in the past, their seizure of Djiboutian territory recently has in our view led to the buildup of arms."
Djibouti hosts French and US military bases and is the main route to the sea for Eritrea's foe and Washington's regional ally, Ethiopia.
France urged Eritrea to be "co-operative" and let a neutral party shuttle between Djibouti and Asmara for talks.
Africa's youngest nation, Eritrea has fractious ties with the West, which accuses it of backing anti-goverment fighters in Somalia and expelling UN peacekeepers on its border with Ethiopia.
'Concocted animosity'Egypt's state news agency MENA reported that an emergency session of the Arab League had called on Eritrea withdraw its forces immediately from the border area, stressing the need to respect Djibouti's sovereignty.
There has been no independent verification of events in the remote border areaand Eritrea has dismissed Djibouti's version as "concocted animosity".
"The Eritrean government ... will under no circumstance get involved in an invitation of squabbles and acts of hostility designed to undermine good neighbourliness," it said.
Djibouti accuses Eritrean troops of digging trenches on both sides of the border in the disputed Ras Doumeira area on April 16, sparking tensions between the two Horn of Africa neighbours.
The neighbours fought for control of the area in 1996 and 1999 and have never held talks to resolve the dispute.