NAIROBI, Kenya — Foreign military forces have joined the offensive against the al-Shabab militant group in Somalia as Kenyan troops advanced toward a rebel stronghold, a Kenyan military spokesman said Sunday.

The spokesman, Maj. Emmanuel Chirchir, said the United States or France, or possibly both, had stepped up airstrikes in the past few days, killing a number of al-Shabab militants. The French navy has also shelled rebel positions from the sea, Kenyan officials said.

The United States and France have not confirmed involvement in Somalia.

If Western military powers have indeed joined the conflict, analysts said, it could represent a turning point against al-Shabab, a ruthless group that has pledged allegiance to al-Qaida.

The group controls much of southern Somalia, though its young fighters and battered pickups are deemed no match for a sophisticated army.

Several U.S. officials contacted Sunday in the United States and Kenya declined to comment. A French diplomat in the United States did not return phone calls.

The U.S. military has previously conducted surgical strikes in Somalia, taking the opportunity to kill terrorism suspects and al-Shabab fighters who were on the run. In 2006 and 2007, the U.S. military cooperated closely with a large Ethiopian force that stormed into Somalia to oust an Islamist movement that had taken control of much of the country.

About a week ago, Kenya sent hundreds of its soldiers into Somalia to battle al-Shabab, whom the Kenyans blame for a recent wave of kidnappings in Kenya.

Many independent analysts, however, doubt the group had a role in the abductions.

Kenya's military says that it plans to remain in Somalia until al-Shabab's capacity is "reduced" and Somalia's weak, U.S.-backed transitional government is able to function.

But Kenya's military — especially compared with its neighbors', like Ethiopia, Uganda, Sudan and Somalia — has scant experience.

Several military efforts over the past 20 years by others, from the U.S. to the United Nations, have failed to deliver a sustainable government in Somalia.

Kenyan military officials say their plan is to squeeze the port of Kismayu, one of Somalia's biggest towns and a major money-earner for al-Shabab, from two sides in a pincer movement. Heavy rains, though, have literally bogged them down, and after an initial burst of activity, the Kenyan advance seems to have slowed considerably.

On Sunday, Kenyan officials said a French naval ship had shelled the city of Koday, south of Kismayu. The French military has also launched small, covert strikes in Somalia in the past, aimed at terrorism suspects and pirates.

A possible reason for French action could be the death announced last week of a French woman who was kidnapped Oct. 1 from a beach bungalow in Kenya and taken to Somalia.

The woman, Marie Dedieu, was a cancer survivor and a quadriplegic, but her abductors refused to allow her to receive medicine, which friends said hastened her death.

Many Kenyans believe the United States is helping Kenya in Somalia. A 2-inch-tall front-page headline in the Sunday Nation, one of Kenya's leading newspapers, blared: "US planes join assault."

Kenya is one of America's ' closest allies in Africa, but last week, U.S. officials said they were caught off guard by the Kenyan offensive and that there were no U.S. ground troops or military advisers involved.

U.S. officials in Kenya are increasingly concerned about the prospect of al-Shabab attacking Kenya's capital, Nairobi, in revenge, and possibly targeting Westerners. Kenya has been struck before by terrorists. Al-Qaida blew up the U.S. Embassy in Nairobi in 1998, killing more than 200 people, and in 2002 a beachfront hotel was bombed, killing more than 10.