The United States has expressed serious concern over the growing number of extrajudicial killings in the Philippines as President Rodrigo Duterte's ambitious war on drugs continues unabated.

Deputy assistant secretary of state for Southeast Asia Patrick Murphy said that Washington was aware of Manila's objective and urged the Asian country to investigate the killings.

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

Since Duterte assumed office in June 2016, an estimated 6,000 people – including drug users and drug dealers – have been killed in the bloody war on drugs. The killings have been denounced by world leaders and human rights activists.

According to reports, the majority of the victims were killed by police officers in self-defence, while several others were killed by paid assassins, who are working for the police.

In January, activists in the US expressed serious concerns when Secretary of State Rex Tillerson evaded questions about extrajudicial killings in the Philippines at his confirmation hearings. Murphy brushed off the criticism and said that there was a difference between being the nominee and being secretary of state.

"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 continued.

Duterte was enraged by the Obama administration's criticism on the killings and threatened to cut defence ties with Washington.

Children look inside a room where seven people were shot and killed by suspected vigilantes in December 2016 at a house storing illegal narcotics in Caloocan city Erik De Castro/Reuters