There was no claim of responsibility for the attack in the predominantly Muslim region, which is wracked by political tensions between clans.

The convoy of vans carrying about 40 people was hijacked in Maguindanao province, about 900km south of Manila, and soldiers found the bullet-riddled bodies of 13 women and eight men.

It is not known if anyone survived the attack. An army and police search for hostages is under way.

The national journalists' union said that at least 10 of its reporters were travelling in the convoy.

Victims' relatives blamed political rivals in the national elections scheduled for May.

Philippine elections are violent in the south because of the presence of armed groups, including communists and Muslim rebels fighting for self-rule in the predominantly Roman Catholic nation.