Manila (AFP) - The United States said Thursday it had withheld a major aid package to the Philippines because of significant concerns about the rule of law under President Rodrigo Duterte, days after he boasted about personally killing people.

The announcement is one of the sharpest responses by the United States to Duterte's drug war, which has claimed more than 5,000 lives in less than six months.

The US embassy said the Philippines had missed out on a second aid grant under the Millennium Challenge Corporation, after an initial five-year package worth more than $430 million expired in May.

"This decision reflects the Board's significant concerns around rule of law and civil liberties in the Philippines," embassy spokeswoman Molly Koscina said in a statement emailed to AFP.

The US government set up the Millennium Challenge Corporation to promote economic growth and reduce poverty around the world.

However countries can only qualify if they "demonstrate a commitment to just and democratic governance, investments in its people, and economic freedom," according to the corporation's website.

The United States has been a vocal critic of the war on drugs, with President Barack Obama in September urging Duterte to prosecute it "the right way".

The criticism has severely strained ties between the longtime allies, which are bound by a mutual defence pact.

Duterte has branded Obama a "son of a whore" and told him to "go to hell", while seeking to establish closer ties with US rivals China and Russia.

The Millennium Challenge Corporation announced on Wednesday that new grants had been given to Burkina Faso, Sri Lanka and Tunisia.

The US embassy's Koscina said the corporation's board decided to withhold a vote on whether to reselect the Philippines because of the rights concerns, although it could still be considered in the future.

"MCC will continue to monitor unfolding events in the Philippines and underscores that all country partners are expected to maintain eligibility, which includes not just a passing scorecard but also a demonstrated commitment to the rule of law, due process and respect for human rights," she said.

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- Duterte's Trump card -

However Duterte has said he is looking forward to relations improving under US president-elect Donald Trump.

Duterte said following a phone call with Trump in early December that he had received encouragement for the drug war.

"He was quite sensitive also to our worry about drugs. And he wishes me well... in my campaign and he said that... we are doing it as a sovereign nation, the right way," Duterte said about his call with Trump.

Philippine police have reported killing 2,086 people in anti-drug operations since Duterte took office on June 30. More than 3,000 others have been killed in unexplained circumstances, according to official figures.

Often masked assailants break into shanty homes and kill people who have been tagged as drug traffickers or drug users. Rights groups have warned of a breakdown in the rule of law with police and hired assassins operating with complete impunity.

Duterte has insisted that police are only killing in self-defence and gangsters are murdering the other victims.

But he has also said he will not allow any police to go to jail if they are found guilty of murder in prosecuting his crime war.

This week Duterte said he had personally killed suspected criminals when he was mayor of a southern city to set an example for police.

In September Duterte also likened himself to Nazi leader Adolf Hitler as he said he was "happy to slaughter" three million drug users.

After condemnation from Jewish groups, Duterte apologised for his Hitler reference but said he was "emphatic" about wanting to kill the millions of drug users.

Surveys have shown a majority of Filipinos overwhelmingly support the charismatic Duterte and his crime policies, accepting his argument that drastic action is required to stop the Philippines from becoming a narco-state.

A fresh survey by the Social Weather Stations released Thursday showed 77 percent of Filipinos were "satisfied" with his performance.