The death toll from a suspected bomb explosion on a bus in the southern Philippines rose to at least 10 with another 34 people wounded, officials said Wednesday.

The Rural Transit bus had picked up passengers, mostly students, and was passing near the main gate of Central Mindanao University in Maramag town in Bukidnon province on Tuesday night when the powerful blast sent shrapnel and debris flying through the vehicle, town safety officer Alejandro Navarro said.

The blast killed 10 passengers and wounded 34 others, many of them seriously, police said.

The suspected explosive, probably a mortar round concealed in a bag, went off in an overheard rack, and many passengers who were standing in the overcrowded bus were wounded in the head, Navarro said.

Investigators were trying to determine the type of blast and were searching for other possible explosives at the site in Maramag, about 535 miles southeast of Manila, police officer Rufina Bayarcal said.

Ralph Quilla, a 17-year-old student who was riding on the bus, said by cellphone that the explosion occurred at the back of the vehicle and wounded him in the arm.

"There was a loud explosion that filled the bus with smoke," Quilla said from a hospital where he and other victims were taken for treatment. "I saw bloodied people and jumped out of a window."

Muslim rebels and extortion gangs have been blamed for past bombings on buses in the country's restive south.

The attacks have continued despite a state of high alert for military and police forces in the south because of sporadic bombings and other attacks by hard-line Muslim insurgents opposed to a new autonomy deal between the government and the Moro Islamic Liberation Front, the largest Muslim rebel group in the country.