Suspected separatist militants bombed a market and exploded a grenade in a cinema in the southern Philippines yesterday, killing four civilians and wounding 48 others, as U.S. special forces arrived there for military exercises.

The dead from the market attack included two young boys and an old woman. They died of their wounds in hospital.

No individual or groups claimed responsibility for the two attacks, but police said the Abu Sayyaf is behind the explosions.

"Most probably... this is a political message," police intelligence officer Major Jose Bayani Gucela said.

Residents in Jolo said a letter purportedly written by an Abu Sayyaf leader was distributed there this week warning of bombings and kidnappings if military operations against them continued.

Hundreds of residents were busy shopping when the bomb exploded around 5.15 am in a market on the island of Jolo. Scores were injured in a stampede that broke out as people fled for cover. Acting local police chief Yusup Asjudi said some are critically injured.

"The blast site was several blocks away from the Jolo mosque in Tulay, said Labour Secretary Habib Jamasali Abdurahman of the Muslim autonomous region. Abu Sayyaf militants use the mosque as a meeting place, the military said.

Earlier, a local school received a letter from the Abu Sayyaf saying students and teachers will be taken hostage if the military does not stop operations, said local military chief Romeo Tolentino.

Jailed Muslim rebel leader Nur Misuari and the Abu Sayyaf were initially blamed for the blast, said Tolentino, adding the extremist group and his supporters had formed a tactical alliance.

The market was one of the areas attacked allegedly by his followers then. Jolo is a stronghold of the Abu Sayyaf and Misuari's followers.

"Soldiers have launched an offensive against the rebels in Jolo," said Southern Command operations chief Col. Roland Detabali.

In Zamboanga, five people were wounded as a grenade explosion ripped through a cinema in the crowded Minpro shopping mall, in the heart of the city, at 5pm, 12 hours after the fatal blast in Jolo island.

A military report identified four of the wounded as Terrybelle Sy, 19; Charlie Marasigan, 40; Catherine Gonzales, 27; and Alma Rosales, 36. One person remains unidentified. Most of the victims suffered slight shrapnel injuries.

The grenade exploded in Cinema B where Lord of the Rings was showing.

"I had to help a wounded woman as she came out of the cinema. She was coated with blood and came to us and said 'I'm wounded, I'm wounded'. There was an explosion, followed by shouts and then a rush of people came out of the cinema," said security guard Jovi Pestano.

The entrance of the cinema was bloodstained and littered with paper, shoes, slippers and edibles. Police sealed the cinema as soldiers led K9 sniffer dogs in. In October last year, 11 people were killed and 50 others were wounded in a grenade blast at Zamboanga City's Peuriculture Centre, a food court.

"We still do not know who was behind the Zamboanga attack," said Detabali.

The attacks will in no way alter the schedule for the U.S. forces, said Michael Farris, a U.S. military spokesman.

"We have our security plans and we are confident of our security plans," he said.

"The exercises will push through," said Colonel Roland Detabali, operations chief for the southern Philippine military command.

The blast will not affect the joint operations with the U.S. in Zamboanga, said Brigadier General Edilberto Adan, adding that Jolo, where the other explosion occurred, is far from Basilan where the joint war games will also be held.

At an air base six km away, U.S. special forces troops were landing at the start of their deployment, Washington's biggest expansion of the war against terror after the campaign in Afghanistan.

The attack in Zamboanga and Jolo came hours before fresh US troops arrived on board an MC-130 cargo plane.

About 200 American logistics and communications personnel are in Zamboanga City. Thirty-two of the 160 men belonging to U.S. special forces will be sent to Basilan today.

At the Zamboanga cinema hall, movie-goers seemed unperturbed by the grenade attack.

By the evening, only a few hours later, the cinema had reopened and people were queuing up for the late show. Only a small gash in the floor showed where the grenade had landed.

The U.S. and Philippine governments say the Abu Sayyaf is linked to the Al Qaida network of Osama bin Laden.