Manila, July 9 -- Amnesty International urgently called for international pressure and an immediate U.N. investigation to help end what it says are possible crimes against humanity in the Philippine president's bloody anti-drug crackdown.

The London-based rights watchdog said in a study released Monday that extrajudicial killings in President Rodrigo Duterte's 3-year-old campaign remain rampant and the scale of abuses has reached "the threshold of crimes against humanity."

About 6,600 people, most of them accused of petty drug crimes, have been killed in the crackdown Duterte launched as his centerpiece project when he took office in mid-2016. But nongovernment groups claim a much higher death toll, including many suspects killed by mot...