At least five Hong Kong tourists were killed during a dramatic hostage siege in the Philippine capital on Monday, doctors said.

Policemen walk towards a tourist bus containing more than 20 Hong Kong adults and children being held hostage by an ex-policeman armed with a high-powered assault rifle, in Manila on August 23.

"At the moment there are five casualties. Four are confirmed dead. One is in a critical condition," Faith Gaerlan, emergency medicine chief at Manila Doctors Hospital, told AFP.

Another hospital also received the body of a woman it identified as among the hostages, a member of its emergency medical staff said.

Metropolitan Manila police chief Director Leocadio Santiago said police could not immediately give an exact figure of the casualties and those who survived.

A total of 22 Hong Kong tourists and three Filipinos were aboard the tourist bus when sacked police senior inspector Rolando Mendoza seized the vehicle early Monday, demanding that he be reinstated.

He released nine people during police negotiations, including two Filipinos.

The remaining Filipino, the bus driver, escaped when Mendoza began shooting at the hostages by nightfall, forcing police to launch an assault in a bid to free the remaining 15 hostages.