WASHINGTON - The United States said on Thursday it was troubled by the growing number of extrajudicial killings in Philippine President Rodrigo Duterte's war on drugs and called on Manila to stick to its commitment to investigate them.

Close to 9,000 people, mostly drug users and small-time dealers, have been killed since Duterte took office almost 10 months ago and promised an unrelenting campaign to rid the Philippines of illicit narcotics.

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Police say about a third of the victims were shot by officers in self-defense. Human rights groups believe many of the remaining two thirds were killed by paid assassins cooperating with the police or by police themselves, disguised as vigilantes. The government and police reject that.

Patrick Murphy, the U.S. deputy assistant secretary of state for Southeast Asia, said the United States shared Manila's objective of eliminating the scourge of illicit drugs and wanted to help.

"We however do have a very sustained and deep concern when elements of the drug war are operating outside the rule of law," Murphy told reporters. "The growing number of extrajudicial killings is troubling."

Rights advocates were concerned when U.S. Secretary of State Rex Tillerson sidestepped questions about extrajudicial killings in the Philippines during his January confirmation hearing, raising the possibility that President Donald Trump might take a softer line on the issue than his predecessor, President Barack Obama.

Murphy said there was a distinction between being a nominee and the secretary of state and Tillerson was now the leader of the policy of expressing concern about the way the drug war was being waged.

"We are urging the Philippines to follow up on its commitment to investigate extrajudicial killings whether they are committed by law enforcement, or of a vigilante nature," he said.

Earlier on Thursday, Duterte's office rejected allegations by two senior police officers in a Reuters report that police received cash rewards for executing drug suspects, while the most high-profile critic of the president backed the officers' claims.

• PNP denies funding drug kills

Duterte was infuriated by US expressions of concern about extrajudicial killings after he took office last year and threatened to sever the long-standing US defense alliance.

Duterte spoke positively about Trump, a fellow populist, after the US presidential election in November, although his anti-US rhetoric continued.