The capital of a southern Philippine island was under police and military lockdown Wednesday after a siege by Al Qaeda-linked militants left 15 people dead. At least two military armoured personnel carriers patrolled the streets of Isabela city on Basilan island a day after Islamist militants from the Abu Sayyaf group set off bombs and attacked civilians. 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,' regional military chief Lieutenant General Ben Dolorfino told selfnews by phone from his base in Zamboanga city, near Basilan. 'Right now, we have two Marine companies and one from the Special Forces guarding the city, or around 300 men,' he said. 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. The city mayor, Cherry Akbar, said 15 people were killed in the violence, including five militants who apparently perished in the first blast. The dead also included three Marines, a policeman and six civilians. Dolorfino said the attack appeared to have been well planned, noting that the police and military uniforms as worn by the gunmen were brand new. He said military intelligence had received information ahead of Tuesday's siege that the Abu Sayyaf was plotting an attack. 'They were planning something big. This was well planned and apparently they were well funded,' Dolorfino said. 'I don't think there was a failure of intelligence on our part because we were pursuing certain leads (ahead of the attack).' 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 selfnews. '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. 