Philippine police forces were on alert on Tuesday, the eve of the anniversary of the September 11, 2001, terrorist attacks in the United States, a spokesman said.

All police units were told to initiate preventive measures against terrorist activities by local groups, Chief Superintendent Nicanor Bartolome said.

"Based on historical data, some terrorist organizations have been known to launch activities to commemorate some dates and occasions that are significant to their organization," he said.

Police forces were ordered to "intensify intelligence operations to monitor and pre-empt movement of known threat groups and strengthen the defences of vital installations and soft targets," he said.

Bartolome said police units have also been placed on standby to respond to emergencies.

He noted the al-Qaeda terrorist network, which was behind the September 11 attacks, has links with Islamic militants in the Philippines.

Al-Qaeda is believed to be providing financial support to the local Muslim Abu Sayyaf rebel group and the regional Jemaah Islamiyah terrorist network, whose top militants are believed to be hiding in the southern Philippines.