MANILA (AFP) - Nearly half of Filipinos believe police are killing innocent people in waging President Rodrigo Duterte's anti-drugs war, according to survey results released on Wednesday (Sept 27).

Police have reported killing 3,850 suspected drug dealers who allegedly resisted arrest during a 15-month crackdown waged in the name of Duterte's election campaign vow to rid society of narcotics.

Thousands of other people have been murdered in unexplained circumstances which rights groups partly attribute to vigilante death squads.

Previous surveys showed popular support for the crackdown. The latest poll by the respected Social Weather Station (SWS) was the first indication that significant numbers of Filipinos doubt the official version of the deaths.

"Forty-nine per cent of the country agrees with the statement, 'Many of those killed by the police in the anti-drug campaign are not really drug pushers'," SWS said in a statement.

Twenty-three per cent disagreed, while 27 per cent were undecided, according to the Manila organisation, which surveyed 1,200 adults face-to-face across the country from June 23-26.

Twenty per cent "strongly agree" and 34 per cent "somewhat agree" that 'Many of those killed by the police in the anti-drug campaign did not really fight against the police", it added.

Eight per cent strongly disagreed and 12 per cent somewhat disagreed, with 25 per cent undecided.

Duterte spokesman Ernesto Abella dismissed the poll findings.

"It seems the 2nd quarter Social Weather Stations survey contains leading and pointed questions that may have unduly influenced the answers of respondents," Abella said in a statement.

National police spokesman Chief Superintendent Dionardo Carlos said the poll findings were about "perception", adding 85 police officers had also been killed and more than 200 wounded while enforcing the crackdown.

It was the first time such a figure had been given.

"It only shows that there was resistance and danger because we lost lives," Carlos added.

Political analyst Richard Javad Heydarian told AFP that while Filipinos backed the drug war, in principle, "they have consistently raised concern with extrajudicial killings as well as the reliability and professionalism of the national police".

He added: "It certainly puts pressure on Duterte to instill more discipline within the law enforcement ranks."

Dindo Manhit, president of the Manila-based policy think-tank Stratbase ADR Institute, said past SWS surveys consistently showed strong support for Duterte and his war on drugs.

"Once the public does not support government programmes it might affect the legitimacy of the whole government, and this is not good for a democratically elected president," Manhit added.

Duterte has rejected international rights monitors' warnings he could be orchestrating a crime against humanity.