MANILA: At least 21 people were murdered in the lawless southern Philippines on Monday in a massacre that the military and relatives of the victims said was likely linked to a political rivalry.

Supporters of a prominent local politician in Maguindanao province and local journalists were among those murdered, with some of them beheaded and mutilated, the military and relatives said.

Armed forces spokesman Lieutenant Colonel Romeo Brawner said the bodies of 21 people had been recovered, 13 of whom were women, and that the death toll was likely to rise.

"We believe more bodies are buried in the ground and we are trying to recover them," Brawner said in an interview with the ABS-CBN television network.

The murders occurred after gunmen linked to Maguindanao governor Andal Ampatuan abducted members of a rival political clan and the local reporters who had been travelling in a convoy on Monday morning, according to Brawner.

The leader of the rival political clan that was attacked, Esmael Mangudadatu, had been meaning to nominate for the governorship of the mainly Muslim Maguindanao province for next year's elections.

He was not in the group of about 40 people that was abducted, apparently after he had received warnings from Ampatuan's people not to register in the polls.

But Mangudadatu's wife was in the group and had been intending to lodge his nomination for him.

The Mangudadatu family is known to have a long-running feud with the Ampatuans, who police say are known to control their own private army.

Mangudadatu said his wife was among those killed and that many of the victims had been mutilated.

"Their private parts were showing, their heads were crushed, they were mutilated," he told ABS-CBN, as he blamed the Ampatuan clan for the killings.

Before the bodies were recovered, Brawner confirmed gunman linked to Ampatuan had abducted the group.

Although he would not say afterwards that the Ampatuan clan was responsible for the killings, he said it was a strong suspect.

"Right now that's the angle we're looking at. The abduction of the Mangudadato family members by the elements of the Ampatuans is due to a political feud," Brawner said.

Revenge killings and clashes among rival political clans are common in Maguindanao and other parts of the strife-torn southern Mindanao island, where unlicensed firearms proliferate and a Muslim insurgency has waged for decades.

The Philippines is also regarded as one of the most dangerous places in the world for journalists.

However, the scale of Monday's slaughter sent immediate shock waves through the country.

"This is a gruesome massacre of civilians unequalled in recent history," said Jess Dureza, President Gloria Arroyo's adviser on Mindanao island.

"There must be a total stop to this senseless violence. I strongly recommend that a state of emergency be imposed in the area and everyone be disarmed. Anything less will not work."

Arroyo's office later released a statement saying no effort would be spared to bring justice to the victims.

"Civilised society has no place for this kind of violence," the statement said.

Amid reports from media groups that as many as 12 of the victims may have been journalists, the National Press Club of the Philippines also expressed outrage.

"We are condemning this brutal incident. We have this culture of impunity in Mindanao that needs to change," club president Benny Antiporda said.

Before the reports of the massacre, Brawner said the leader of the militiamen who staged the kidnapping was one of Ampatuan's sons.

Esmael Mangudadatu's brother, Khdadafeh, also said Ampatuan had warned Esmael not to register for the elections.

"His son, Andal Ampatuan Jnr, is supposed to run for governor and he had already made an earlier announcement that we would be killed if (Esmael) filed the candidacy for governor," Khdadafeh told AFP.