Ethiopia says its forces have attacked a military base inside arch-rival Eritrea, in what Ethiopia calls an act of retaliation.

Government spokesman Shimeles Kemal said Ethiopian troops moved 16 kilometers into Eritrea early Thursday and launched what he called a “successful attack” against two military posts used by “subversive groups.”

He said the groups are sponsored by the Eritrean government, and have carried out attacks inside Ethiopia.

Shimeles, speaking to reporters in Addis Ababa, indicated there was no fighting between the Ethiopian and Eritrean armies but gave no further details on the operation.

An Eritrean diplomat contacted by VOA said he had no response to the incident.

The United States has urged both sides to exercise restraint. U.S. State Department spokeswoman Victoria Nuland said Washington is seeking further clarification from Ethiopia about its intentions.

Ethiopia has repeatedly accused Eritrea of backing rebel groups that attack targets in Ethiopia's Afar area. Shimeles specifically mentioned an incident in January in which five European tourists were killed.

Ethiopia and Eritrea fought a war between 1998 and 2000 that killed an estimated 80,000 people. Tension between the countries has remained high ever since.