Communist guerrillas and a Muslim rebel group have denied planting a bomb that killed at least six people in a crowded bus station in the southern Philippines.

The military has named both the Communist New People's Army (NPA) and the Moro Islamic Liberation Front (MILF) as key suspects in the blast, which happened in Kidapawan city on Wednesday.

The bomb was placed by unidentified men under a concrete bench in the terminal, said local police chief Superintendent Casimiro Medes.

One woman and a child were killed at the scene, and the others died in hospital, police said.

The powerful bomb - which police initially said was a grenade - injured about 25 people and damaged two buses. Police have confirmed that six people died, correcting earlier statement that eight had been killed.

Denials

MILF spokesman Eid Kabalu condemned the attack, saying it was "un-Islamic and those responsible should be punished."

He also said the MILF would have nothing to gain from such an attack as the group had a ceasefire agreement with the government.

NPA spokesman Gregorio Rosal, speaking on radio, said his group had nothing to do with the bombing and that the military was trying to falsely label the group as terrorists.

Police are working on the theory that the bombers were trying to extort "protection" money from the Weena bus company.

The management of the company said it had received several letters demanding money, including one signed by a "Suicide Bomber Team".

US soldier killed

Police have been deployed on Weena buses for several months following previous bombings, said Superintendent Medes.

Wednesday's attackers may have left the explosives at the terminal because security was too tight on the buses, he said.

The attack in North Cotabato province came a week after a US serviceman was killed in a bomb in the southern city of Zamboanga.

The authorities blamed that attack on another Muslim group, the Abu Sayyaf, which is better known for kidnapping for ransom.

The Abu Sayyaf had warned a week earlier it would mount attacks on civilian, military and US targets to retaliate for the army offensive against Muslim rebels in the southern Philippines.

The MILF is the main group demanding a Muslim homeland in the south of the mainly Catholic Philippines.