Philippine National Police Chief Ronald Dela Rosa told a Senate committee on Monday that 712 drug traffickers and users had been killed in police operations since July 1.
Police were also investigating 1,067 other drug-related killings, Dela Rosa said, without giving details. On Sunday, Duterte railed against the United Nations for criticising the wave of deaths.
The United States, a close ally of the Philippines, said it was "deeply concerned" by the reports, and US State Department spokesman Mark Toner urged Duterte's government to ensure that law-enforcement authorities abided by human rights norms.
The drug trafficking crackdown and some strongly worded criticisms Duterte has made of the United States since coming to power present a dilemma for Washington, which has been seeking to forge unity among allies and partners in Asia in the face of an increasingly assertive China, especially in the strategic South China Sea.
Toner made the dilemma clear in responses to questions at a regular State Department briefing in Washington, in which he referred to Duterte as "a plain-speaking politician."
"We continue to make clear to the Philippines government ... our concern about human rights, extrajudicial killings, but we are also committed to our bilateral relationship and strengthening that bilateral relationship," he said.
Toner said there was no question of the United States turning a blind eye to rights abuses and that the relationship with Manila, while good, was "frank and candid."
As recently as Sunday, the number of suspected drug traffickers killed in Duterte's war on drugs had been put at about 900 by Philippine officials. But this number included people who died since Duterte won the May 9 presidential election.
Duterte said in a strongly worded late-night news conference on Sunday the Philippines might leave the United Nations and invite China and others to form a new global forum, accusing it of failing to fulfil its mandate.
His foreign minister, Perfecto Yasay, said on Monday the Philippines would remain a UN member and described the president's comments as expressions of "profound disappointment and frustration".
"We are committed to the UN despite our numerous frustrations and disappointments with the international agency," Yasay told a news conference.
US officials declined comment on Duterte's UN remarks.
Last week, two UN human rights experts urged Manila to stop the extra-judicial executions and killings.
Yasay said Duterte has promised to uphold human rights in the fight against drugs and has ordered the police to investigate and prosecute offenders.
He criticised the UN rapporteurs for "jumping to an arbitrary conclusion that we have violated human rights of people".
"It is highly irresponsible on their part to solely rely on such allegations based on information from unnamed sources without proper substantiation," he said of the United Nations.
Senator Leila de Lima, a staunch critic of the president, started a two-day congressional inquiry into the killings on Monday, questioning top police and anti-narcotics officials to explain the "unprecedented" rise in killings.
"I am disturbed that we have killings left and right as breakfast every morning," she said.
"My concern does not only revolve around the growing tally of killings reported by the police. What is particularly worrisome is that the campaign against drugs seems to be an excuse for some law enforcers and other elements like vigilantes to commit murder with impunity," De Lima said.