MANILA, Philippines — Three American senators asked the US Department of State to explain the use of funding aid in the Philippines to make sure the money is not being used for the government's war on drugs.

US Senators Edward Markey (D-Massachusetts), Marco Rubio (R-Florida) and Christopher Coons (D-Delaware) have expressed grave concern over alleged extrajudicial killings and human rights violations in the country under the President Rodrigo Duterte's anti-narcotics campaign.

"The Philippine anti-drug movement known as Project [Tokhang] in fact appears to be a campaign of mass atrocities thinly disguised as a response to a public health emergency," the senators said in a letter addressed to US Bureau of International Narcotics and Law Enforcement Affairs Assistant Secretary William Brownfield.

The senators noted that instead of addressing the drug problem, investing in treatment programs or approaching the issue with an emphasis on health, Duterte has "pledged to kill another 20,000 to 30,000 people, many simply because they suffer from a drug use disorder."

The senators stressed that drug addiction is not a moral failing but a chronic disease.

Citing the United Nations (UN) World Drug Report, Markey, Rubio and Coons noted that more than 29 million people are now living with a drug use disorder.

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Earlier this year, the UN had identified the need for a balanced approach to drug control which prioritizes the health and well-being of people, families and communities.

"Duterte's anti-drug campaign does just the opposite, and exhibits an abhorrent response to the public health crisis of drug addiction," the senators' letter read.

The senators requested the US Department of State to explain the process for tracking the use of US funds in the Philippines to ensure that none of their foreign assistance money is being allocated to law enforcement activities supporting the war on drugs.

The senators also asked what work the US government is doing to remedy the drug crisis in the Philippines and to help people recover from addiction.

"Is the US directly or indirectly supporting access to treatment and other services for those dealing with a substance use disorder? Do Philippine law enforcement services, which we support, work directly with health agencies on these matters?" the senator asked Brownfield.

Markey, Rubio and Coons requested the State Department to check that it is ensuring full compliance with the Leahy Law in relation to its assistance to the Philippines in light of Duterte's campaign against drugs.

The Leahy Law prohibits the US Department of State and Department of Defense from providing assistance to foreign military units that violate human rights. In September, US Sen. Patrick Leahy (D-Vermont), author of that law, also raised concerns on human rights.

"[W]hen governments condone extrajudicial killings and forced disappearances, and prey on vulnerable populations, they are sowing the seeds of instability, not preventing it," he said then.

The senators maintained that the use of law enforcement can help monitor and control illegal drug sales but stressed that extrajudicial killings are not a form of justice.

"We urge the US to denounce these horrific violations of basic human rights, and ensure that no foreign assistance is being provided to support egregious acts against humanity," the senators said.

RELATED: US moves aid away from Duterte's drug war

Earlier this year, the US State Department put a planned sale of assault rifles to the Philippine National Police on hold over human rights concerns. This month, the US foreign aid agency Millennium Challenge Corporation deferred voting on an aid package to the Philippines due to rule of law and rights issues. It may vote on the package again in March 2017.

The aid package would have followed a five-year $434 million poverty reduction program that it funded during the previous administration.