ZAMBOANGA, Philippines, November 9, 2009 (AFP) - The Philippine government vowed Monday to take revenge against Al Qaeda-linked Islamic militants after they dumped the severed head of a kidnapped school principal at a petrol station.

The head of Gabriel Canizares was found inside a bag at the petrol station on the restive southern island of Jolo at dawn, 22 days after he was abducted, but the rest of his body remained missing, local police said.

The Abu Sayyaf had demanded a P2-million ($42,000) ransom for the release of Canizares, but authorities and his relatives refused to pay.

President Gloria Arroyo's office said the Abu Sayyaf, blamed for the country's worst terrorist attacks and other beheadings of kidnap victims, was behind Canizares' murder, and vowed tough action against the militants.

"We shall make them pay for the enormity of this savagery," Arroyo's spokeswoman, Lorelei Fajardo, said in a statement.

She said authorities were determined to "put an end to the Abu Sayyaf group's heinous and inhumane atrocities".

Arroyo had ordered the military and police units operating on Jolo and other Abu Sayyaf strongholds in the southern Philippines into "full swing" in an effort to crush them, according to Fajardo and the military.

"A full-scale military and police manhunt has been launched to go after the criminals responsible for this barbaric act," armed forces chief General Victor Ibrado said, but he gave no details on how many troops were involved.

Small numbers of US military forces have been in the southern Philippines for the past eight years to train local soldiers in how to fight the Abu Sayyaf, but the group has continued to cause major security problems.

Dozens of Filipino soldiers as well as militants have been killed in clashes on Jolo and nearby islands over the past year alone.

On September 29, two US soldiers and a Filipino marine were killed in a roadside bomb planted by the Abu Sayyaf in the Jolo town of Indanan.

The murder of Canizares threw the spotlight on the Abu Sayyaf and the US military's efforts just three days before US Secretary of State Hillary Clinton was due to visit the Philippines.

The Philippine government had previously said security issues would be discussed during Clinton's two-day trip.

The Abu Sayyaf was set up in the 1990s allegedly with seed money from Osama bin Laden's Al-Qaeda network, and is listed by the United States as a terrorist organisation.

It has been blamed for many of the Philippines' worst terrorist attacks, including the bombing of a ferry in Manila Bay that claimed more than 100 lives in 2005, and the abduction of American tourists in 2001.

One of those Americans was beheaded while another was killed in a military rescue.

While the Abu Sayyaf wants to set up an independent Muslim homeland in the south of the Catholic Philippines, authorities also regard its members as bandits who engage in crimes such as kidnappings to raise money.

The Philippine military says there are only 300-400 Abu Sayyaf militants, although the group enjoys strong local support from Muslim communities on Jolo and nearby islands.