UNITED NATIONS — The first African Union troops arrived in Somalia yesterday at a moment that neighboring countries, the United Nations, and America say could be crucial for the future of the war-ravaged country.

The thirty-five soldiers who, according to a U.N. official, landed at Baidoa are an advance team of a force that is expected to grow to 8,000 A.U. troops.

The U.N. Security Councilbacked A.U. force in Somalia will eventually replace the Ethiopian troops that in December defeated the Qaeda-affiliated Islamic Courts Union, which ruled the country for six months.

Because Somalia is in the Horn of Africa, commerce in the eastern part of Africa and the oil-rich Middle East could be disrupted if the country again comes under the control of Islamic terrorists. According to a report last year that was partially confirmed by a U.N. official yesterday, the ICU maintains ties with other Middle Eastern terrorist groups.

President Museveni of Uganda, which is expected to lead the A.U. force in Somalia, told the troops yesterday that they will not be asked to disarm the local militias. “Once we empower the Somalis, if they think that the guns should be removed from the population, they will be the ones to do it,” he said, according to the Associated Press.

But the U.N. humanitarian coordinator for Somalia, Eric Laroche, told reporters at the United Nations yesterday that the ICU fighters had only retreated after their defeat by the Ethiopian army in December.

Mr. Laroche, who in the past has served in Afghanistan, said he had talked to the Taliban and, more recently, to representatives of the ICU. In both cases, “I got exactly the same language,” he said. The Islamist militia leaders said “they are going to retreat, and then they are going to come back.”

Last year, a U.N. Security Council-sponsored team reported that at least 700 Somalis had traveled to Lebanon, where they fought alongside Hezbollah in its war with Israel in August. That report was received with skepticism at the time, but yesterday Mr. Laroche confirmed that a nephew of a member of his U.N. staff had gone to Lebanon. “They haven’t gone there to fight,” he said. “They went there to train.”

According to the Security Council report, several neighboring countries have supported militias in Somalia.

“I would like to take this opportunity, especially, to request member states who have contacts with the leaders of the ICU, especially those in Yemen, to encourage them” to join the political process, the U.N. special representative in Somalia, Francois Fall, told representatives of Muslim countries who met in Mogadishu last week.

After the ICU was chased out of the Somali capital in December, a political process was launched to try to unite the country under a transitional government. U.N. representatives have reported that Somalis are “exhausted” after years of fighting, but they also warn that without a strong military presence, the militias will return. According to Mr. Laroche, 8,000 troops are not enough to maintain security.

Other U.N. sources say, however, that Ethiopian troops could secretly remain in the country even after Ethiopia transfers power to the African Union.

Meanwhile, according to the Navy Times, U.S. Navy ships are patrolling off Somalia’s shores, where reports of piracy have increased recently.