Ten of the victims died when gunmen ambushed a military convoy near the town of Maimbung.

Troops were sent to engage the rebels and during the battle which followed 10 more soldiers died.

The military blamed Abu Sayyaf - an Islamic militant group allegedly tied to al-Qaeda - and rogue elements in the Moro National Liberation Front (MNLF).

The MNLF, which signed a peace deal with the government in 1996, said�it was behind Thursday's ambush.

It said the attack was retaliation for an army offensive a day earlier.

Hatimil Hassan, deputy chairman of the MNLF, said on local TV: "(The ambush) was not Abu Sayyaf. It was our troops. It was the military's fault. They started it all."

The government wants peace with the MNLF, although some of its members have refused to disarm.

But Manila has sworn to destroy Abu Sayyaf, which is blamed for the Philippines' worst terror attack - a ferry bombing in 2004 that killed more than 100 people.

Thousands of Philippine troops are based in Jolo, some 950km (600 miles) south of the Philippine capital, Manila.

They have been fighting Islamic militants holed up in the island's mountainous terrain for several months.