The Philippines' Catholic Church will assail President Rodrigo Duterte's war on drugs for creating a "reign of terror" among the poor, in sermons to be read at services across the country this weekend, three church sources told Reuters on Saturday.

In its most strongly worded attack so far on the crackdown on drug pushers and users, the Catholic Bishops' Conference of the Philippines will say that killing people is not the answer to trafficking of illegal drugs and will voice concern about the indifference of many to the bloodshed.

"An additional cause of concern is the reign of terror in many places of the poor. Many are killed not because of drugs. Those who kill them are not brought to account," they say in a pastoral letter, a copy of which was obtained by Reuters.

More than 7,600 people have been killed since Duterte launched his anti-drugs campaign seven months ago, more than 2,500 in what police say were shootouts during raids and sting operations.

Both the government and police have strenuously denied that extrajudicial killings have taken place during the campaigns.

The president's office had no immediate comment on the bishops' letter.

Beginning with Mass services this evening, priests will read out the letter signed by the bishops of Asia's biggest Catholic nation.

Their address will not mention Duterte by name, but will urge "elected politicians to serve the common good of the people and not their own interests" and call for steps to tackle "rogue policemen and corrupt judges".

Nearly 80 pe rcent of the Philippines' 100 million people are Catholic and, unlike in many other countries where the faith has waned, the majority still practise with enthusiasm.

While that support historically has given the Church significant political and social clout, it has been hesitant to criticise the blunt-spoken president's current war on drugs.

In interviews with Reuters last year, more than a dozen clergymen said they were uncertain how to take a stand against the killings given the popular support Duterte's campaign enjoyed.

Some said challenging him could be fraught with danger.

Duterte has routinely attacked the Church. He cursed the Pope for causing traffic snarl-ups during his 2015 visit, and as recently as this week called for a "showdown" with priests whom he has accused of having wives, engaging in homosexual acts, misusing state funds and molesting children.

In an apparent reference to accusations that many drug pushers and users have been victims of extra-judicial killings, the bishops said in their pastoral letter "every person has a right to be presumed innocent until proven guilty" and the law should be followed.

"We must also give priority to reforming rogue policemen and corrupt judges," the letter said. "We also call upon elected politicians to serve the common good of the people and not their own interests."

The bishops' letter comes less than a week after Duterte suspended all police operations in the drug crackdown due to what he said was deep-rooted corruption. He has put an anti-drugs agency in charge of the campaign and says he wants the armed forces to play a supportive role.

Amnesty International said in a report this week that police prosecuting the war on drugs had behaved like the criminal underworld they were supposed to be suppressing, taking payments for killings and the delivery of bodies to funeral homes. It said the wave of drugs-related killings appeared to be "systematic, planned and organised" by authorities and could constitute crimes against humanity.

In a series of reports last year, Reuters showed that the police had a 97-percent kill rate in their drug operations, the strongest proof yet that police were summarily shooting drug suspects.

The Reuters reports also found that low-level officials in poor neighbourhoods helped police assemble "watch lists" of alleged drug users and pushers that were effectively hit lists, with many of the people named ending up dead. Duterte used exaggerated and flawed data, including the number of drug users in the Philippines, to justify his anti-narcotics crackdown, according to a Reuters investigation.