Philippine police have killed a town mayor in his jail cell in a purported gun battle with officers, the second killing in a week of a politician linked to illegal drugs under President Duterte's crackdown.

Key points: Police say they found guns, suspected drugs in Espinosa's cell

Police say they found guns, suspected drugs in Espinosa's cell Presidential palace says Espinosa's death "unfortunate"

Presidential palace says Espinosa's death "unfortunate" Another mayor linked to drugs was killed in shootout with police last week

Mayor Rolando Espinosa Sr of Albuera town in central Leyte province and a fellow inmate Raul Yap were shot dead before dawn after they fired at officers who staged a raid in search of firearms and illegal drugs, police said.

"As a matter of procedure, this incident will undergo investigation to establish the facts and circumstances surrounding the incident," regional Police Chief Superintendent Elmer Beltejar said.

Police said they recovered a .45 calibre pistol and a .38 Super pistol from the cells of Yap and Espinosa.

They said a small sachet containing suspected methamphetamine and assorted drug paraphernalia were also found inside Espinosa's cell.

An anti-crime watchdog has called for an investigation of the circumstances of the killings, wondering how the slain mayor and fellow inmate got hold of guns and what prompted them to clash with several policemen while in detention.

The presidential palace described Espinosa's death as "unfortunate" and said an investigation is ongoing.

Second mayor to die in gun battle

More than 3,600 people have died since the start of Rodrigo Duterte's drug crackdown. ( Reuters: Kim Kyung-hoon, file )

Last week, police killed another town mayor — Samsudin Dimaukom — and nine of his men, also allegedly in a gun battle in the southern Philippines.

Espinosa and Dimaukom were among more than 160 officials named publicly by President Rodrigo Duterte in August as part of a shame campaign.

Espinosa's son, an alleged drug lord, was arrested in Abu Dhabi last month.

After being linked by Duterte to illegal drugs, Espinosa surrendered to the national police chief in August in a nationally-televised event.

He had publicly denied any part in the drug trade but said his son was peddling "shabu" (methamphetamine), which he gets from a jailed Chinese drug trader.

Espinosa was later released but was arrested last month after being charged on drug and illegal possession of firearm charges.

Thousands killed in Duterte crackdown

Police estimate that more than 3,600 suspected drug dealers and users have been killed since Mr Duterte took office on June 30.

Many of those killed in the initial months of the crackdown were poor drug suspects, and police said "high-value targets" including mayors and drug lords, would be their next target in a new phase of the crackdown that was launched last week.

The unprecedented crackdown and killings have helped considerably ease crimes, but US officials led by President Barack Obama and other Western governments, along with human rights watchdogs, have been alarmed and called for an end to the killings.

One human rights advocate has called the killings a "human rights calamity".

However, Mr Duterte has lashed out at Mr Obama and other critics, saying he is dealing with a pandemic that has afflicted politics, corrupted even generals and threatened to turn the country into what he describes as a "narco state", similar to some Latin American countries.

Reuters/AP