MANILA, SEPT. 22 -- Communist rebels in the Bicol region south of the capital have stepped up attacks in recent days, and officials said today that they feared the activity may mark a new campaign as rebels seek to take advantage of the disarray in the military in the aftermath of last month's aborted coup.

The rebels have bombed three bridges, hijacked a passenger train and destroyed portions of a major railroad track. Government officials in Bicol said the attacks could be aimed at isolating the region from the rest of Luzon Island and also cut off land access to the island of Samar, where communist activity is reported to be widespread. One official in Bicol, who asked not to be quoted by name, today said he believed more than one-third of the region is now under communist influence.

Gen. Fidel Ramos, the armed forces chief of staff, today said the communists in Bicol were taking advantage of "the temporary reduction of the armed forces' capability" caused by the Aug. 28 military mutiny. "They see that they have new opportunities," Ramos said in a lunch with foreign correspondents. As many as 2,000 soldiers took part in the coup attempt, attacking the presidential palace and several military facilities in Manila and the provinces. There is widespread sympathy in the armed forces for grievances aired by the coup leader, who is still at large.

Since the coup attempt, Ramos said, the communists have stepped up their attacks nationwide and have increased the military's daily casualty rate from an average of three to more than four a day.

Officials said the communists could be mounting the attacks to embarrass the military and the government of President Corazon Aquino. Bicol is the region that gave Aquino her largest margin of victory over Ferdinand Marcos in the 1986 snap presidential election.

Luis Villafuerte, the acting governor of Camarines Sur province, where the rebels have been active, said military estimates show that about 2,000 armed communist rebels now operate in Bicol. He said the recent attacks amount to a "direct confrontation" with the local government authorities and the Catholic church.

Resource-rich Camarines Sur also supplies some of the power needs for Manila and the rest of Luzon island.

Villafuerte, a former Aquino Cabinet member, said the rebel offensive began after military officials received reports of a major arms shipment due to arrive in the region.

The attacks in Bicol appear to signal a shift in tactics by the communist New People's Army, which in the past has concentrated on military and government installations and avoided attacks on economic targets that might inconvenience the citizens.

"The rebels have launched a new phase of economic warfare, hoping to create a situation of strategic stalemate," said Sen. Victor Ziga, who is from Bicol. Ziga and other Bicol politicians said the communists could be hoping to force the local government officials into some form of negotiation.

In the most dramatic of the attacks Sunday, several hundred rebels wearing fatigues hijacked a passenger train en route to Manila from Naga City. The passengers were forced off, and the rebels commandeered the train between Manila and Del Gallego. They killed at least five policemen, two civilian volunteer troops, and a civilian noncombatant, officials said. The rebels apparently searched for other police or military officials before reboarding the train to escape.

They covered their escape by dynamiting one span of a key railroad bridge, according to reports reaching here, which were confirmed today by Bicol political leaders.

Last week, the rebels tore up about 40 yards of the main railway line between Manila and Bicol and cut transmission lines of a government power company. Villafuerte said today that power had returned to normal.

Local elections for thousands of mayors, town councilmen and governors around the country are tentatively scheduled for January. In his speech today, Ramos said the Communist Party of the Philippines and its armed wing, the New People's Army, were making a strong push to consolidate and expand their position in the localities.

The 25,000-member NPA is now believed to operate in at least 68 of the country's 73 provinces, according to most estimates.

"They are making a strong bid to gain control at the local level," Ramos said. By contrast, he said, local government -- and the lack of a working, efficient infrastructure to deliver government services to the provinces -- remains the primary weakness in the Aquino administration's counterinsurgency campaign.