MANILA, Philippines - Government forces on Wednesday placed highly urbanized cities in Basilan province on heightened alert after a siege by Qaeda-linked militants left 12 people dead in Isabela City.

Lt. Gen. Benjamin Dolorino, chief of the Armed Forces' Western Mindanao Command, said military intelligence had received information ahead of Tuesday's siege that the Abu Sayyaf was plotting an attack, similar to the infamous 1995 massacre in Ipil, Zamboanga Sibugay.

"They might be planning something big, similar to what happened in Ipil," he told ABS-CBN's "Umagang Kay Ganda."

On Apr. 4, 1995, at least 200 armed bandits robbed banks, plundered stores, burned down homes, took hostages and massacred more than 50 people.

Dolorfino said that before Tuesday's attack, authorities monitored circulating text messages that an Ipil-like attack will be launched by the Abu Sayyaf in an urbanized area.

At least 25 Abu Sayyaf militants wearing police and military camouflage uniforms on Tuesday set off two bombs that blew up a van and damaged a Roman Catholic church, in the worst attack by the group in months.

A third bomb placed near a judge's house and a bus terminal was safely detonated by soldiers.

The bombings sparked gunbattles around the city, with the militants targeting helpless civilians scampering to safety.

Dolorfino said that the plan to launch a major terrorist attack might still be in play.

He said police and military forces in urbanized areas in Mindanao, particularly in Basilan and Zamboanga City, have been placed on heightened alert.

"It could happen in other urbanized areas in Basilan and other areas, like Zamboanga City. This is why we are on heightened alert in urbanized areas," Dolorfino said.

At least two military armored personnel carriers patrolled the streets of Isabela City a day after the attack. Road blocks were also set up in and around the impoverished city of 150,000 people, where some shops remained closed for business a day after the violence.

"We are trying to get the situation back to normal, but we are implementing heightened security there now. Right now, we have two Marine companies and one from the Special Forces guarding the city, or around 300 men," Dolorfino said in a separate interview.

Basilan Bishop Martin Jumoad condemned the attack and called on his flock to remain calm.

"We condemn this inhuman attacks in the strongest possible terms," Jumoad told AFP. "We appeal for the public to remain calm because God will not fail us."

He said this was the first time that the Santa Isabel Cathedral had been targeted since it was built in 1970.

"The church is 70 percent damaged. We won't be able to hold mass here temporarily," he added.

The Abu Sayyaf is a small gang of Islamic militants on the US government's list of foreign terrorist organisations.

It was founded in the 1990s with seed money from Osama bin Laden's Al-Qaeda network to fight for an independent Muslim state in the south of the mainly Catholic nation, Philippine military intelligence officials say.

The most brutal of several armed Muslim groups in the south, the Abu Sayyaf is also blamed for the country's worst terrorist attacks, including a 2004 bombing of a ferry that killed over 100 on Manila Bay.  With a report from Agence France-Presse