MANILA, Philippines -- The Armed Forces of the Philippines said on Wednesday it deployed a fresh batch of US-trained Army elite force to terror-stricken Basilan to pursue the Abu Sayyaf bandit group that set off bombs on the island on Tuesday, and secure vulnerable districts from further attacks.

A team of Scout Rangers and a Light Reaction Company from Zamboanga City had "sealed off" downtown areas in Basilan as of Wednesday, while Navy vessels started patrolling the waters off the island to stop the bandit group from escaping to other provinces, the Navy spokesperson, Lt. Col. Edgard Arevalo, told reporters at Camp Aguinaldo on Wednesday.

At least 15 people were killed, including the brother of Abu Sayyaf leader Purudji Indama, in Tuesday's series of attacks staged by armed men clad in military and police uniforms, which authorities had identified as Abu Sayyaf members.

Arevalo said on Wednesday that government troops continued to track down Indama's group since Tuesday night after text messages of Abu Sayyaf?s plans to carry out more attacks on the island circulated.

"We are not letting our guards down," said Arevalo, adding that troops have also started securing two vulnerable areas-- Maluso town and Lamitan City, where intelligence units have spotted two small Abu Sayyaf groups, including Indama.

"They are no longer in Isabela City... the two groups have been sighted in the outskirts of Lamitan," confirmed Arevalo.

The latest sightings of the Abu Sayyaf group believed to be responsible for the attacks on Tuesday were already the focus of intensified operations by government troops on the island, he said.

"We don't want to alarm the residents but these are the areas we are looking at, which the Abu Sayyaf could make as subjects of their attack," said Arevalo.

The military has been aiming to prevent the two groups from linking up into a bigger force, he added.