ZANZIBAR, Tanzania, Dec. 24 — Ethiopia officially plunged into war with Somalia’s Islamist forces on Sunday, bombing targets inside Somalia and pushing ground troops deep into Somali territory in a major escalation that could turn Somalia’s internal crisis into a violent religious conflict that engulfs the entire Horn of Africa.

The coordinated assault was the first open admission by Ethiopia’s Christian-led government of its military operations inside Somalia, where — with tacit American support — it has been helping a weak interim government threatened by forces loyal to the Islamic clerics who control the longtime capital, Mogadishu, and much of the country.

Ethiopia’s prime minister, Meles Zenawi, said in a televised broadcast that he had ordered the action because he had no choice.

“Ethiopian defense forces were forced to enter into war to protect the sovereignty of the nation,” he said. “We are not trying to set up a government for Somalia, nor do we have an intention to meddle in Somalia’s internal affairs. We have only been forced by the circumstances.”

According to witnesses, Ethiopian fighter jets bombarded several towns, obliterating an Islamist recruitment center and other targets, while Ethiopian tanks rolled into battle. The attacks set off riots in Mogadishu, Somalia’s battle-scarred seaside capital, and fighting on several fronts in southern Somalia.

Ethiopia, which commands the region’s most powerful military, did not disclose how many troops, tanks or planes it had sent into Somalia, but the United nations has said at least 8,000 Ethiopian soldiers may be in the country. Casualties were reported Sunday, but reliable estimates were impossible to ascertain.

Until now, Ethiopian officials had denied that they had any combat forces inside Somalia, saying instead that their involvement was limited to a few hundred military advisers.

Over the past few months, the Islamist clerics in Somalia have threatened Ethiopia for supporting their rivals, the internationally recognized transitional government.

On Saturday, after several days of heavy internal fighting, Islamist leaders announced that Somalia was now open to Muslim fighters around the world who wanted to wage a holy war against Ethiopia, a country with a long Christian history, even though it is about half Muslim.

“What did you expect us to do?” said Zemedkun Tekle, a spokesman for Ethiopia’s information ministry. “Wait for them to attack our cities?”

Even before Ethiopia’s escalation on Sunday, there were alarming signs that the conflict in Somalia could quickly spiral out of control. According to United Nations officials, at least 2,000 soldiers from Eritrea, which recently waged war with Ethiopia, are fighting for the Islamists. They have been joined by a growing number of Muslim mercenaries from Yemen, Egypt, Syria and Libya who want to turn Somalia into the third front of holy war, after Iraq and Afghanistan.

On Friday, residents of Mogadishu said they saw boatloads of armed men landing on the city’s rocky beaches. On Sunday, after the bombings, Islamist leaders boasted of bringing in more. Still, from the Ethiopian government’s viewpoint, the bombings may be delivering at least some of the desired effect.

For the first time since the Islamists came to power in Somalia in June and rapidly began expanding their reach, they seemed to be losing ground. In at least three places on Sunday — Idaale, Jawil and Bandiiradley — transitional government troops were pushing the Islamists back.

Residents of Beledweyne, near the Ethiopian border, said that after the Ethiopian jets pounded several armed pickup trucks belonging to the Islamists, the rest of the Islamist soldiers fled to the hills.

The bombs also destroyed a recruitment center and a fuel depot, killing at least 10 people, witnesses reported. Hours later, the transitional troops marched into the area, and a new mayor was installed.

Many of Beledweyne’s people seemed relieved, not so much about the change in government, but because the fighting appeared to have ended so fast.

“We’re so sick of war,” said Ahmed Issa, a shopkeeper in Beledweyne. “We’ll obey anybody.”

Much of Somalia has been mired in anarchy since 1991, when the central government collapsed, setting off a long, nasty interclan war. While the United Nations and donor countries struggled to get a new government on its feet, a grass-roots movement of Islamic courts began to gain power.

After Islamist leaders defeated the last of Mogadishu’s warlords, they immediately restored a sense of law and order unheard of in the capital for 15 years. Then they began pushing outward, eventually reaching the outskirts of Baidoa, the seat of the transitional government.

The transitional government has never been popular, and its military forces are divided between rival politicians, many of whom spend the majority of their time outside of Somalia. This summer, Ethiopia began slipping soldiers across the border to protect both the transitional government and Ethiopia itself.

The Islamists had threatened to liberate Somali-speaking areas of Ethiopia and stir up Ethiopia’s Muslim population.

American officials acknowledged that they tacitly supported Ethiopia’s approach because they felt it was the best way to check the growing power of the Islamists, whom American officials have accused of sheltering terrorists tied with Al Qaeda. A State Department spokesperson in Washington said Sunday that the United States was assessing reports of the surge in fighting in Somalia but provided no further comment.

A major question going forward seems to be whether Ethiopian forces will advance into Mogadishu and try to finish off the Islamist military, a possibility that many fear could spur a long and ugly insurgency, or simply deal the Islamists enough of a blow to force them back to negotiations with the transitional government.

The rival authorities in Somalia have flirted with the idea of sharing power, but several rounds of peace talks have produced little but broken promises.

In a hint of a possible direction to come, Ethiopia’s prime minister recently told American officials that he could wipe out the Islamists “in one to two weeks.”

But many analysts fear that the presence of Ethiopian troops in Somalia will only make matters worse, because of the history of conflict between Ethiopians and Somalis. The two nations have battled over contested border areas before, and the difference of religions, with Somalia almost purely Muslim, has often been an aggravating factor.

On Sunday, as word began to spread that Ethiopian planes were bombing Somalia, students in Mogadishu rushed into the streets and began rioting. They kicked in doors and smashed plate glass windows, yelling at the few shopkeepers still open: “This is not time for business! This is time for war!”

The Islamists are using teenagers as their main fighting force. Last week, right after heavy combat began between the Islamist troops and the transitional government forces, Islamist leaders closed all schools in Mogadishu to funnel more young people into battle.

Witnesses in frontline areas have said that waves of young, poorly trained Islamist fighters have been mowed down by Ethiopian troops. Ethiopia’s military is trained by American advisers and is supplied with millions of dollars of American aid.

On Sunday, Abdulrahim Ali Modei, the Islamists’ information minister, conceded at a news conference that many of the Islamist troops had been killed, but he did not sound discouraged.

“These are victories,” he said. “Our soldiers are in paradise now.”

Yussuf Maxamuud and Mohammed Ibrahim contributed reporting from Mogadishu, Somalia.