Isabela (Philippines): A powerful booby trap bomb wounded 22 Philippine soldiers, seven of them critical, as they patrolled a former stronghold of Al-Qaeda-linked militants on Tuesday, the military said.

The soldiers were patrolling the outskirts of a remote camp captured last month from Abu Sayyaf extremists when the device went off, local army commander Colonel Ricardo Visaya said.

The Abu Sayyaf had formerly used the camp on the southern island of Basilan, to hide many of their kidnap victims until a large military assault dislodged the rebels in March, Visaya said.

"The camp had a lot of improvised explosive devices planted around it... to strengthen (the extremists') defensive position. They are very difficult to detect," he said.

He described the device as an old booby trap left behind by the Abu Sayyaf before they abandoned the camp and was set off when soldiers tripped on it.

Helicopters were deployed to airlift the wounded to a military hospital, Visaya said.

The heavily-forested island of Basilan is a known stronghold of the Abu Sayyaf, a group founded with seed money from Al-Qaeda chief Osama bin Laden in the 1990s.

The group has been blamed for the worst terror attacks in Philippine history and has frequently resorted to kidnapping to raise funds, often targeting foreigners.

Seven foreigners a Dutchman, a Swiss national, an Australian, two Malaysian traders, an Indian and a Japanese man are believed to still be held by the Abu Sayyaf and other outlawed groups in the south.

US troops have been based in the southern Philippines for a decade to help train local soldiers in hunting the Abu Sayyaf.