Kidnappers in a lawless part of the Philippines have freed an American woman after more than two months in captivity, but held on to her son and nephew, according to authorities.

Gerfa Yeatts Lunsmann, her 14-year-old son Kevin and her Filipino teenage nephew were abducted on July 12 on a resort island offZamboanga city, close to other southern islands where a range ofIslamist militants are based.

Lunsmann's ordeal ended on Sunday night after the kidnappers took her by boat from an undisclosed location to a wharf on nearby Basilan island, and ordered her to walk into a town, local officials and the military said.

However her son and nephew remain in captivity, said Celso Lobregat, mayor of Zamboanga city.

"Efforts are now being made to also recover the children," Lobregat told AFP, but did not give further details.

Regional military spokesman Lieutenant Colonel Randolph Cabangbang said Lunsmann was taken to a military base in Zamboanga to recover and for health checks, but no details about her medical condition were released.

No group has officially claimed responsibility for the kidnapping although Islamist militants are suspected.

A Muslim insurgency in the south that has dragged on for more than 40 years has given birth to a wide range of Islamist militants, bandits and pirates, many of which regularly carry out kidnappings for ransom.

The most infamous of the groups is the Abu Sayyaf, which numbers only a few hundred militants but is on the US government's list of foreign terrorist organisations.

Hundreds of US soldiers have been stationed in the southern Philippines for a decade to train the local military to fight them.

The Abu Sayyaf is blamed for a series of kidnappings, including the abduction of 20 people from a Philippine island resort in 2001.

An American tourist among them was beheaded in captivity while one of two US missionaries kidnapped at the same time was killed during a rescue mission.

Shortly after the kidnappings of Lunsmann and the two boys local reports in Zamboanga quoted unnamed officials as saying the abductors had demanded $10 million for their release, although this was never confirmed.

The US embassy in Manila issued a statement confirming Lunsmann's release and thanking local authorities for securing her freedom, but made no mention of whether a ransom was paid.

"This outcome could not have occurred without the concerted efforts of Philippine government officials and the personal engagement of Mayor Celso Lobregat of Zamboanga City," the statement said.

"We commend the diligent, professional and effective efforts of everyone involved in securing (her) release."

Lunsmann, who was born in the Philippines, gained American citizenship after being adopted by a US couple as a child. She married a US-born German, who was the father of their son abducted along with her.

Lunsmann was abducted while visiting a resort that she had been developing near the area where she was born. She had reportedly returned to her home town repeatedly over the years.