MOGADISHU, Somalia — Fighters belonging to a militant Islamist movement fled into a rugged, forested corner of Somalia from rapidly advancing government forces yesterday, and the prime minister offered amnesty if they surrendered.

Regional diplomats worked to arrange the speedy deployment of African peacekeepers to help the interim government establish its authority in the country, which has known chaos for 15 years.

As the last remaining stronghold of the Islamic group was overrun by government troops backed by Ethiopian tanks and MiG fighter jets, the net began closing on suspected Al Qaeda militants believed to be sheltered by the hardline group.

Neighboring Kenya vowed to seal its frontier to prevent any extremists, now wedged against the sea and their border, from escaping the 13-day military offensive.

Sea routes from southern Somalia were also being patrolled by the U.S. Navy, hunting for three Al Qaeda suspects believed to be among the Islamic group and wanted for the 1998 bombings of American embassies in East Africa.

“Kenya cannot be a haven for people who are not wanted by their lawful government,” Kenyan government spokesman Alfred Matua said.

Thousands of Somalis have fled their homes in the wake of advancing Ethiopian and government forces, but most have returned to their homes once the fighting subsided. The U.N. refugee agency has sent supplies to the Kenyan border as a precaution, but so far, there has been no increase in the number of Somalis seeking refuge in Kenya, the country director for Kenya for UNICEF, Christian Balslev-Olesen, said.

The military advance marked a stunning turnaround for Somalia’s government, which just weeks ago could barely control one town — its base of Baidoa — while the Council of Islamic Courts controlled the capital and much of southern Somalia.

But with the intervention of Ethiopia, which has one of Africa’s largest armies, the Islamic group has been forced from the capital of Mogadishu and other key towns in the last 10 days. Its casualties run into the thousands, Ethiopia said.

Yet it does not mark the end of the Islamists or their ultimate defeat. The group has promised to wage an Iraq-style guerrilla war if defeated, and a woman was killed Sunday in a mysterious blast in Mogadishu.

Diplomats want the peacekeeping force to replace the muscle of Ethiopia, a Christian country long despised in Islamic Somalia. Both countries have fought two wars, the last in 1977, and Somalia lays claim to territories in Ethiopia.

In a bid to cement its control, Prime Minister Gedi of Somalia ordered a nationwide disarmament beginning today, an immense task in a country awash with weapons after more than a decade of civil war.

“The warlord era in Somalia is now over,” Mr. Gedi said at a news conference in the recently captured capital, giving a three-day deadline for handing over all weapons. Somali warlords, who have begun returning to Mogadishu after the Islamists’ defeat, have not yet voiced agreement.

Many here believe the only chance for real stability in Somalia lies with international peacekeepers — not with the government.

“There is a power vacuum already,” according to peace activist Ali Said Omar, speaking in gun-infested Mogadishu. “Everybody has taken his own weapons back. How can the government say it’s in control?”

The Islamic forces in the coastal stronghold of Kismayo began to disintegrate after a night of artillery attacks at the front line and following a mutiny within their ranks, witnesses said. They fled to a base near the Kenyan border on a small peninsula called Ras Kamboni, where there is a pier for traditional oceangoing boats known as dhows.

Ethiopian MiG fighter jets flew low, searching for boats that might be carrying escaping Islamic fighters.

Just six months ago, the Islamic group defeated an American-backed alliance of Somali warlords controlling Mogadishu and then swept through much of southern Somalia.

With them came a semblance of order in this chaotic country — but also a strict and often severe interpretation of Islam, which raised memories of Afghanistan’s Taliban.

Somalia’s interim government and its Ethiopian allies have long accused Islamic militias of harboring Al Qaeda, and the American government has said the 1998 bombers have become leaders in the Islamic movement in Africa.

“If we capture them alive, we will hand them over to the United States,” Mr. Gedi said.

Islamic movement leaders deny having any links to Osama bin Laden’s Al Qaeda network.