WASHINGTON (CNN) -- The U.S. State Department is urging Americans to be cautious while traveling in Ethiopia because of a tense border dispute with neighboring Eritrea.

U.S. citizens already in the East African nation "should continue to monitor the situation and periodically review their personal security," a public announcement from the State Department said Thursday. Travel near or across the Ethiopia-Eritrea border "should be avoided," the announcement said.

In early May, the long-running border dispute boiled over with hundreds of people being killed, and fighting did not subside until the onset of the rainy season in mid-June. The two sides have since used the lull to mass reinforcements of troops and artillery along their 620-mile (1,000-km) frontier. Both sides also have shown little progress in solving the dispute peacefully and have accused each other of thwarting peace efforts.