Police in the Philippines tightened security after three bombings on Monday killed at least nine people and injured 100 more.

The Muslim extremist group Abu Sayyaf claimed responsibility for the blasts in the capital Manila and two other cities, saying the attacks were a Valentine's Day "gift" to President Gloria Macapagal Arroyo.

"You can attribute this to us," an Abu Sayyaf leader, Abu Solaiman, told DZBB radio after the first two bombings. He added there was "one more to come."

Police investigators inspect the scene where a bomb exploded inside a bus in the financial district of Makati, Philippines. (AP photo)

The group said the bombings were retribution for a military offensive against Islamic militants in the southern Philippines.

Some 60 people have been killed in fighting during a government attack on the island of Jolo.

"Our latest operations – planned and executed with precision by the gallant warriors of Islam – is our continuing response to the Philippine government's atrocities committed against Muslims everywhere," said Solaiman, promising more attacks in the future.

The Manila bombing occurred about 7:30 p.m. local time on a bus travelling along a busy highway. Police said three people were killed and at least 60 others injured.

A blast outside the Gaisano mall in southern General Santos city an hour earlier killed at least five people and injured at least 36 others.

Another bomb that went off almost simultaneously killed a 12-year-old boy and injured at least eight people at a bus terminal in Davao, also in the southern Philippines.

"These are despicable acts of terror, and we ask the people to brace themselves against these attacks on our freedom and security," said presidential spokesperson Ignacio Bunye.

National police head Gen. Edgar Aglipay ordered the 114,000-member police force to tighten security in vital installations and commercial establishments countrywide.