DIGOS CITY, Philippines—At least six persons were confirmed dead while 30 others -- including a girl estimated to be two years old and a pregnant woman -- were wounded when an improvised bomb ripped through a packed passenger bus in this southern Philippine city at 2:45 p.m. Monday.

The blast, caused by an improvised explosive device, tore through the Metro Shuttle Bus Co., which was parked at the public terminal here, police said.

It was not immediately clear how many passengers were aboard the bus or how many people were at the terminal at the time of the explosion.

Senior Superintendent Cesario Darantinao, Davao del Sur police chief, said the explosion was quite powerful that it wrecked the bus and nearly blew off its roof.

"It was so powerful that the blast decapitated a civilian," said Eastern Mindanao command spokesman Major Armand Rico.

As of this posting, only three of the six fatalities were identified -- Marlon Doey, estimated to be between 30 and 40 years old and a resident of Sta. Maria, Davao del Sur; Wella Timotea; and Virginia Flores. Doey was identified by the police. Davao del Sur Douglas Cagas identified Timotea and Flores.

Many of those injured were in critical condition and were rushed to the government-run Davao Medical Center (DMC) in Davao City.

Most of the victims sustained head injuries.

The non-aircon bus bound for Malita, Davao del Sur (not Bansalan as reported earlier) had just come from Davao City and was waiting for passengers when the bomb exploded.

Darantinao said the bomb was placed on the overhead baggage compartment.

"It was remotely detonated through a mobile phone," he said.

But authorities were still trying to determine the exact component of the bomb by examining the fragments from the bus, he said.

A correspondent of the Philippine Daily Inquirer at the scene saw blood littered on the floor of the ill-fated bus.

A witness said black smoke billowed from the bus while terror-gripped passengers tried to get out of the wreckage.

"People also frantically ran away from the terminal," the witness who declined to give his name said.

Stores at the public terminal were immediately closed while authorities cordoned off the area.

Buses passing by the city were made to temporarily use the Davao-General Santos highway as a drop off point.

Mayor Arsenio Latasa said the attack came even as strict security had already been implemented in the city.

Latasa said two police officers, who were about to check on the bus after it arrived, narrowly missed the explosion.

Darantinao said they were suspecting that the extortion group Al-Khobar was behind the latest bomb explosion.

The Al-Khobar, which is also being blamed for dozens of attack on other bus companies plying the southern and central Mindanao regions, was said to be an ally of the Abu Sayyaf, which has been linked to the Jemaah Islamiyah.

"What is sad here is that it has occurred the Muslim world is celebrating Ramadan," Latasa said.

Governor Cagas said the attack was unfortunate but he called on the residents not to be cowed by acts of terror.

Speaking over a local radio here, Cagas urged residents not to panic and instead help authorities catch members of the Al-Khobar.

Tagum City Mayor Rey Uy, whose family owns the Metro Shuttle Bus, said they had no other suspect but the extortion gang, which has been pestering them for protection money.

Uy said the extortion gang, which he declined to identify, had already warned them of more attacks after the July 24 explosion unless the company would give money.

But he said they repeatedly ignored the demand.

"When you give in, they will become bolder," he said.

Rico said troops had been sent to the area to set up roadblocks along possible escape routes in a bid to catch the culprits.

The gang is composed mostly of foreign Muslim rebels and preys on businesses in the south.

A Metro Shuttle official said last month that the gang had demanded P50,000 (about $1,000) a month in extortion money, threatening attacks if it was not paid.

Darantinao said that members of the gang were believed to be responsible for another bomb attack on a Metro Shuttle bus last month. That attack, also in Digos, killed three people.

"We had filed cases against them for multiple frustrated murder" in relation to the earlier bombing, Darantinao said, without disclosing the names of the suspects.

"These are most likely the same people based on description of the modus operandi," he said.