Mogadishu, Somalia ? Ethiopian jets pounded Islamic-held positions in southern Somalia, a sharp escalation Sunday of a conflict that diplomats fear could ignite a regional war.

Several hundred people already have been killed in the past five days in fighting between Ethiopian forces and Somalia’s Islamic militias.

Witnesses and officials said early-morning strikes by Ethiopian military planes killed about 80 fighters and civilians and wounded another 300 in the town of Baladweyne, which has been held by Somalia’s Islamic Courts Union.

“The enemy launched full-scale war against Somalia,” said Sheik Mohamoud Ibrahim Suley, an official of the Islamic courts. “The fighting has commenced, and it will not stop unless Addis Ababa stops the aggression.”

Ethiopia’s Prime Minister, Meles Zenawi, went on national television to say the country had been “forced to enter into war … to blunt repeated attacks by Islamic courts terrorists and anti-Ethiopian elements they are supporting.”

Ethiopia until now had acknowledged only sending several hundred military advisers to Somalia.

International diplomats warn that Somalia’s worsening strife could ignite a full-scale war in the Horn of Africa, the region in the northeastern part of the continent which lies across the Red Sea and Gulf of Aden from Saudi Arabia and Yemen.

Somalia has been without a functioning government since 1991. The courts union, an alliance of Islamic religious leaders, seized control of Mogadishu and most of southern Somalia this summer and has been battling the weak transitional federal government for control of the rest of the country.

Ethiopia, a largely Christian country whose leaders are fearful of an Islamic takeover in Somalia, is backing the transitional government. A third country in the region, Eritrea, has been backing the Islamic militias. The U.S. government, which has worked with Ethiopia’s military in the past, has worried that the courts union could provide sanctuary to Islamic radicals.

Sunday’s attack came one day after the Islamists’ top security officer called on Muslims worldwide to come to the assistance of Somalia in what they are characterizing as a “holy war” against Ethiopia.

In Washington, a State Department spokeswoman, Leslie Phillips, said Sunday that U.S. officials had seen the reports of escalated fighting in Somalia, “and we’re following the situation closely.”

She noted that in recent weeks, the U.S. has recognized that Ethiopia “has a security concern” in Somalia, but has “urged all parties to act with restraint.”

Eric Laroche, the United Nations Humanitarian Coordinator for Somalia, urged all sides to cease the hostilities in order to protect thousands of Somali refugees fleeing the violence.

“Indiscriminate shelling is a clear violation of the laws of war and has a devastating impact on the most vulnerable segments of the country,” Laroche said. “People must have the option to move from areas of active conflict.”