Six Philippine soldiers who were killed in one of the bloodiest clashes against Al-Qaeda-linked militants this year were hit by friendly fire, the military acknowledged today.

The soldiers were hit by artillery rounds fired by another military unit as troops battled Abu Sayyaf militants on the strife-torn southern island of Jolo on June 19.

"These are the realities of combat. Factors like combat stress, the fog of war, affect decision-making," military spokesman Lieutenant-Colonel Ramon Zagala told AFP.

The unit that fired the howitzer rounds said it was "definitely not intentional", but a military inquiry was underway to determine accountability, Zagala added.

The military had earlier said the fighting left seven soldiers dead, six of whom were killed by mortar shells launched by Abu Sayyaf.

Twenty-four other soldiers were wounded, many of them from the shelling, according to the original military report.

The Abu Sayyaf, founded in the early 1990s with seed money from bin Laden's Al-Qaeda network, is blamed for the deadliest and most brutal terrorist attacks in the Philippines' recent history.

These include the 2004 bombing of a ferry that left more than 100 dead, abductions of foreign missionaries and tourists, and beheadings mostly of local people.

Several foreign and local hostages are still believed to be held by the Abu Sayyaf in the jungles of Jolo.

The campaign against them is backed by about 500 US military advisers who have been rotating through the southern Philippines since 2002 to train Filipino soldiers.

Many Abu Sayyaf leaders have been captured or killed and the US announced last month that it would pull out the bulk of the American troops this year.

However the militants continue to pose a threat in the area, kidnapping people and carrying out bomb attacks.