Philippine President Rodrigo Duterte has signed into law a bill giving the police chief and two other senior police officials the power to issue subpoenas to hasten the investigation of crimes.

The law "will add more teeth to their mandate to enhance the law and find solutions to criminal cases", spokesman Harry Roque told a news briefing on Saturday.

Human rights activists expressed concern about the new law, saying the Philippine police force was notorious for abuses and could use the additional power to trump-up criminal charges against those critical of Duterte's administration.

Subpoena law

Under the new law, such subpoena power would rest exclusively with the Philippine National Police (PNP) chief and the director and the deputy director for administration of the PNP's Criminal Investigation and Detection Group.

"The subpoena shall state the nature and purpose of investigation, shall be directed to the person whose attendance is required" states the law, which Duterte signed on March 1.

Roque said not all police would have subpoena powers and the issuing of a subpoena would not mean automatic detention.

"This subpoena power will give hope to the many victims of crimes who were deprived of justice due to the slow investigation processes as witnesses or respondents to crimes cannot be forced to face investigation," he said.

Human rights

Women's welfare advocacy group Gabriela described the law as "a green light to more extrajudicial killings, arbitrary arrests, and trumped-up charges against dissenters and opposition".

"We stand firm that the PNP should have no quasi-judicial or prosecutorial investigative powers, especially with its undisputed record of corruption, arrogance, human rights violations, and its fondness for legal shortcuts," the group said in a statement.

One of Duterte's main campaign promises during the 2016 presidential election was to wipe out crime and illicit drugs by using deadly force.

About 4,000 mostly urban poor Filipinos have been killed by police in the past 20 months in a brutal crackdown against illegal drugs that has alarmed the international community. Activists believe the death toll is far higher.