Mogadishu, Somalia ? Hundreds of Ethiopian troops in armored vehicles rolled into Somalia on Thursday to protect their allies in this country’s virtually powerless government from Islamic militants who control the capital.

The move could give the U.S.-backed Somali government its only chance of curbing the Islamic militia’s increasing power. But Ethiopia’s incursion also could be just the provocation the militia needs to build public support for a guerrilla war.

“We will declare jihad if the Ethiopian government refuses to withdraw their troops from Somalia,” said Sheik Sharif Sheik Ahmed, a top Islamic official.

The neighboring countries are traditional enemies, although Somali President Abdullahi Yusuf Ahmed has asked Ethiopia for its support. Thousands of Somalis have taken to the streets in recent weeks to denounce witness accounts of Ethiopian troops along the border.

The United States urged Ethiopia to exercise restraint and said the U.S., European Union, African Union, Arab League and others in an international contact group on Somalia would meet soon to consider the volatile situation.

Somalia has been without an effective central government since warlords toppled dictator Mohamed Siad Barre in 1991 and then turned on each other, carving much of the country into armed camps ruled by violence and clan law.

The government, which includes some warlords linked to the violence of the past, was established with the support of the United Nations to help Somalia emerge from anarchy. But the body wields no real power, has no military and only operates in Baidoa, about 100 miles east of the Ethiopian border.

The Ethiopians, wearing their national military uniforms, deployed Thursday at the airport outside Baidoa and set up a fenced compound near the transitional president’s home in the city, witnesses said.

The Islamic militia of the Supreme Islamic Courts Council stepped into the power vacuum in recent months, seizing the capital of Mogadishu and most of southern Somalia.

On Wednesday, the militia reached within 20 miles of Baidoa, prompting the government to go on high alert.