Military spokesman Lieutenant Colonel Daniel Lucero said about 50 guerrillas of the New People's Army (NPA) attacked an airforce detachment securing a power plant in Calaca town, Batangas province, before dawn.

Plant security men and soldiers said they stopped the rebels from breaking through the plant's defences and were pushed back after a heavy 15-minute exchange of fire, but a nearby army post was nearly overrun.

"The power plant is secured and operational," said Dennis Gana, a spokesman for the National Power Corporation that operates the 600 megawatts coal-fired thermal plant.

Lucero said the raid was intended to knock out the power plant which is a crucial part of the electrical grid serving the main Philippines island of Luzon, including Metropolitan Manila. 

First major offensive

The attack was the first major offensive by the NPA since a Christmas season ceasefire, separately called by both the government and the rebels, expired earlier this month. 

The communists had rejected an appeal by the government to extend the ceasefire until the 10 May presidential and national elections. 

The New People's Army (NPA) has been waging a protracted guerrilla war to install a Maoist-led state.

The military says the number of rebels fell to 8,800 at the end of 2003 from 9,200 a year earlier but their influence remains strong in remote villages in the east of the archipelago, from Luzon island in the north to Mindanao in the south.