Hundreds of suspected drug dealers and addicts have been killed in the Philippines since President Rodrigo Duterte essentially declared open season on them after his election in May. But with international outrage mounting over the wave of extrajudicial killings, keeping track of the exact death toll has become increasingly challenging.

On Monday, National Police Chief Ronald dela Rosa told the country's Senate that more than 1,900 drug suspects have been killed in the past seven weeks. Although cited extensively in media reports, that number stands in stark contrast to a running tally of confirmed deaths maintained by the Philippine Inquirer. According to the newspaper's "kill list," there have been 712 drug-related killings since May 10, the day after Duterte was elected.

According to Sanho Tree, director of the Washington-based Drug Policy Project, the 1,900 figure includes more than 700 confirmed killings by police officers, as well as hundreds of additional murders committed by vigilantes.

"It's the difference between the police count of police-involved shootings, or shootouts, as they call them, versus vigilantes, death squads, or individual assassins leaving signs saying, 'I'm a drug dealer,' on the corpse, but no other evidence," Tree said. "So we don't know who's doing the killing in that regard."

In response to growing methamphetamine use in the Philippines, Duterte has offered to award medals and bounties to citizens and police officers who kill drug suspects.

Tree says it's possible some of the killings attributed to vigilantes may have been committed by police, but "police have been reporting their kill counts — they haven't been embarrassed about it, so I don't know why they wouldn't report more of them."

Tree also noted that off-duty police officers may be involved in killing drug suspects.

"It could be like in Latin America, as we've seen over the decades, particularly in the '80s and '90s. Social cleansing was done by paramilitary death squads, or by police after hours," he said. "They would take off their uniforms and put on a black shirt, and presto, you're the paramilitaries and no one is willing to identify you. You can get away with murder that way."

Tree helped organize a protest against the killings on Tuesday outside the Philippine embassy in Washington, and similar demonstrations were held in New York and San Francisco.

After two United Nations human rights officials issued a statement recently saying drug suspects in the Philippines should be "judged in a court of law, not by gunmen on the streets," Duterte called them "stupid" and threatened to withdraw the country from the international organization.