At least 13 "high-value" detainees facing drug charges have escaped from a jail in northern Philippines, the latest in a series of prison breaks in the country, according to authorities.

The escapees slipped out of the jail in a police camp in San Fernando City, north of the capital, Manila, before dawn on Sunday.

"They sawed through the bars of the metal grille," Derrick Arnold Carreon, spokesman for the Philippine Drug Enforcement Agency, told the AFP news agency, adding that an investigation was under way to find out how they got the saw.

IN PICTURES: Inside overcrowded prisons in the Philippines

It was not clear how the prisoners obtained the metal-cutting materials, and how they got past security at the prison gates.

The Philippine Daily Inquirer website reported that the escapees were inmates from the province of Bulacan, and have been in jail for at least five months.

The website quoted Juvenal Azurin, an anti-drug agency official, as saying that the detainees are considered "high-value targets, because of the volume of illegal drugs seized" from them.

Azurin was also quoted as saying that a manhunt operation is being carried out in Manila and other nearby provinces.

Prisons in the Philippines are usually overcrowded, poorly maintained and inadequately guarded - and mass escapes are frequent.

In the country's biggest jailbreak, more than 150 inmates escaped a prison in the southern Philippines in January after about 100 gunmen stormed the facility.

Most of the inmates were later re-arrested.

In August 2016, members of a Muslim armed group that pledges allegiance to the Islamic State of Iraq and the Levant (ISIL) group stormed a jail in the south and released 23 inmates.