American singer-songwriter James Taylor has cancelled a concert in the Philippines in a show of opposition to what he said were "unacceptable" killings outside the judicial process as part of the country's war on drugs.

The Grammy award-winning artist apologised to fans for scrapping his February concert in Manila, saying in a statement that he agreed with dealing with drugs offenders under the law, but was against punishing them outside it.

"I don't think of my music as being particularly political but sometimes one is called upon to make a political stand," said Taylor, 68, whose songs include "You've Got a Friend" and "Fire and Rain".

"For a sovereign nation to prosecute and punish, under the law, those responsible for the illegal trade in drugs is, of course, understandable, even commendable," he said.

"But recent reports from the Philippines of summary executions of suspected offenders without trial or judicial process are deeply concerning and unacceptable to anyone who loves the rule of law."

More than 6,000 people have been killed since no-nonsense President Rodrigo Duterte took office in July and launched a fierce crackdown. Police say a third of those deaths were suspected drug dealers killed during counter-narcotics operations.

The remainder are still under police investigation and are widely attributed to vigilantes, some of which activists believe are hired assassins working for the authorities.

Duterte vehemently rejects that. He also says he is not responsible for Filipinos taking the law into their own hands and will condone vigilantism if it means curbing the country's methamphetamine problem.

Taylor, who has won five Grammy awards since his self-titled debut album came out in 1968, said planned performances in Singapore, Hong Kong, Australia and New Zealand would go ahead as scheduled.