Cotabato City, Philippines - A homemade bomb was found Friday in an elementary school in the southern Philippines, where security has been stepped up in the aftermath of intense fighting between government forces and Muslim rebels.

The bomb was discovered by civilians in one of the buildings of the public school in Mlang town in North Cotabato province, 930 kilometres south of Manila, said Major Armand Rico, a regional military spokesman.

"Many pupils were already at the school," he said.

The bomb, which was safely defused, was the third to be found in North Cotabato in two days.

On Thursday, a suspected bomber was hurt when the device he was planting in a public market in Mlang prematurely exploded.

In nearby Kidapawan City, authorities defused a powerful bomb made from a 60-millimetre mortar shell packed with 2 kilograms of TNT explosive powder and 1 kilogram of nails and attached to a mobile phone as the detonator. It was planted near the public market.

Rico said the device found in the Mlang school was similar to the Kidapawan bomb.

He said authorities suspected Moro Islamic Liberation Front (MILF) rebels were behind the attempted bombings in retaliation for the military's successful offensive to drive them from 15 villages they had occupied in North Cotabato.

Troops cleared the villages Wednesday after days of fighting that killed 53 people, mostly MILF rebels. Nearly 160,000 residents were displaced by the hostilities, and about half of them have started to return to their homes.

International aid agencies, such as the Red Cross and the World Food Programme, have sent relief goods for those who have chosen to stay longer in evacuation centres.

Felipe Donoso, head of the International Committee of the Red Cross (ICRC) delegation in the Philippines, said the refugees needed food and items to create basic shelters, such as tarpaulins, water containers and mosquito nets.

"The ICRC and the Philippine Red Cross are loading trucks in Cotabato City to distribute food rations," the committee said in a statement Friday. "Water and sanitation kits ... are due to arrive in Cotabato City today."

The hostilities in North Cotabato flared last week after the Supreme Court stopped the signing of a Muslim homeland deal between the MILF and the Philippine government, which would have expanded an existing six-province autonomous Muslim region.

Catholic politicians charged that the deal violated the constitution and would have ceded sovereignty over the southern Philippines to the rebels. They have urged the Supreme Court to declare the agreement unconstitutional. (dpa)