The US State Department said "killings by security forces" were among the "most significant" human rights issues in the Philippines last year.
"Concerns about police impunity increased significantly following the sharp increase in police killings," the US State Department said in its Reports on Human Rights Practices for 2017.
"The government investigated a limited number of reported human rights abuses, including abuses by its own forces, paramilitaries, and insurgent and terrorist groups," the annual report that reviews the human rights environment in at least 195 countries read.
In March, the Philippine Drug Enforcement Agency said 4,075 drug-linked individuals have been killed while 91,704 anti-illegal drug operations have been carried out since President Rodrigo Duterte rose to power in July 2016.
Out of the total 2,467 drug-related homicide incidents recorded, 1,752 are still under investigation, while 715 have been solved, police said.
But the State Department noted that "the reported number of alleged extrajudicial killings varied widely, as government and nongovernmental organizations (NGOs) used different definitions."
Other human rights issues cited in the State Department's report were "harsh prison conditions," and "cases of apparent government disregard for legal rights and due process."
In a statement, Foreign Affairs Secretary Alan Peter Cayetano maintained that the anti-narcotics campaign is "guided by the rule of law embodied in our Constitution, which also enshrines the country’s long-standing tradition of upholding human rights."
"We do not need others who think they know better than us Filipinos to tell us what to do. As a sovereign nation, the Philippines deserves the same kind of respect we have been extending to our friends in the international community," he said.
Cayetano earlier slammed the European Parliament for adopting a resolution that called on the Philippines to halt its drug war.