Suspected Muslim rebels in the Philippines have killed 13 people and injured at least nine in the southern of island of Mindanao.

The attackers ambushed a truck belonging to a Canadian mining company in Zamboanga del Norte province, killing three guards and nine employees - all Filipinos - a military spokesman said.

It was not clear who was behind the attack but Lieutenant Colonel Daniel Lucero said he suspected guerrillas from the Muslim separatist Moro Islamic Liberation Front (Milf).

At least 16 people were killed in a bomb attack in Mindanao on Tuesday.

Police in the Philippines have been on high alert throughout the Christmas period due to fear of attack by another militant group, the Communist New People's Army.

Despite declaring a unilateral four-day truce with the group, the government feared possible attacks on Thursday - the 34th anniversary of the founding of the Philippine Communist Party.

Extortion?

Thursday's attack occurred in Baliguian, 800 kilometres (500 miles) south of the capital, Manila.

Police sources said the mining company - Toronto Ventures Inc. - had previously received extortion demands from the Milf, the Associated Press news agency reported.

And military officials said troops had been sent to hunt the gunmen in an area where Milf rebels are known to be active.

The government feared attacks by Communist rebels

Police had already deployed about 100 plainclothes officers to track Milf members following Tuesday's bombing in which local mayor Saudie Ampatuan was among those killed.

That attack was in the town of Datu Piang, where a mortar was detonated by remote control as the mayor walked past. The Milf denied any responsibility.

The southern Philippines has been a scene of separatist conflict for 30 years, led by Muslim rebels in the predominantly Catholic country.

The Milf have signed a ceasefire and peace talks with the government are expected to resume in Malaysia next month.

The group denies any connection to the more militant Abu Sayyaf, which says it is fighting for a Muslim homeland, but which seems to engage primarily in kidnapping for profit.