(Adds details)

MANILA, June 15 (Reuters) - At least nine people were killed and an undetermined number wounded when a crude bomb exploded inside a packed commuter bus in the southern Philippines on Friday, a police chief said.

Sherwin Butil, police chief in Bansalan town on the southern island of Mindanao, said the bomb went off as the bus was leaving a terminal at about 6 p.m. (1000 GMT).

"No group has claimed responsibility for the attack," Butil told reporters. "We're still investigating whether Muslim militants or criminal groups engaged in extortion were behind the bombing."

Butil said the bomb was placed inside a backpack and left under a seat a few rows behind the driver, who was among the wounded.

"Our investigators were still gathering fragments of the explosive device but some passengers told us they saw two men hurriedly got off the bus holding a device that appeared to be a remote control or a mobile phone," Butil added.

Thirty minutes earlier, a bomb went off in another bus at a terminal in Cotabato City. There were no casualties because all the commuters had left the bus before the blast.

The bus was a wreck, police said, adding a bus company official showed them a letter from a certain Barumbado demanding 2 million pesos ($43,000) as protection money.

Police said three people were killed and 35 wounded when a crude bomb exploded at the same bus terminal in Cotabato City on May 18. That attack was blamed on a criminal gang.

On June 8, at least 10 people were wounded when two explosions ripped through a bus owned by the same transport company in Matalam, also on Mindanao island.

AlertNet news is provided by