MANILA, July 24 (Xinhua) -- Twenty-seven people were injured, four in critical conditions, after a home-made bomb exploded inside a passenger bus in a southern Philippine city Thursday noon.

    Provincial police chief Cesario Darantinao said the improvised explosive device, placed under the seats went off at around 11:50 a.m. and partially damaged the long distance bus parked in a terminal in Digos city of Davao del Sur province.

    "The bomb was detonated by a cell phone. Nails were found at the scene. The blast did not blow up the bus but windows were shattered and the bus floor was partially destroyed," Dearantinao said.

    Darantinao confirmed to Xinhua that police at the blast scene and the hospital found no death at the moment, correcting an earlier statement of a military spokesman who said at least three had been killed.

    Darantinao said the injured, mostly suffering burns and cuts, were sent to Davao Medical Center for treatment.

    Bus companies in the southern Philippines are often subject to extortions of local bandits and armed rebels who in recent years took to explosion as a means of conveying the threat.

    Derantinao said the police "found some extortion activities" linked to Thursday's bus blast but could not immediately confirm on the suspicion. No group has so far claimed responsibility of the blast.

    It was the second bus explosion in the city within two years. The authority said that a local group, believed to have ties with the Southeast Asia terrorist group Jemaah Islamiya (JI) was behind the previous explosion in 2007.