Philippine President Rodrigo Duterte's campaign to bring back the death penalty for drug-related crimes has cleared a major hurdle, with supporters backing it in congress, though critics have denounced the plans as "inhumane".

The death penalty bill, along with a proposed measure to punish children as young as nine as adult criminals, are key planks of Mr Duterte's controversial drug war that has already claimed more than 6,500 lives.

A majority of politicians in the lower house of congress passed a second reading of the bill last night, clearing one of the biggest obstacles in proponents' plans to make the death penalty legal by May.

A third and final reading still needs to be held next week, although with no more debates both sides agree passage is a formality.

Then the Senate, which is similarly dominated by Mr Duterte's allies, would pass a counterpart bill.

"We have hurdled the most difficult part," said congressman Reynaldo Umali, a sponsor of the bill.

Opponents voiced anger the Philippines would bring back the death penalty, 11 years after it was revoked, highlighting among many concerns a corrupt justice system that would lead to innocent people being executed.

Rodrigo Duterte won presidential elections last year

"The decision is inhumane, shameful and blatantly disrespectful," Father Jerome Secillano, executive secretary for public affairs at the Catholic Bishops' Conference of the Philippines, said in a statement.

"Let me reiterate this, criminals should be punished and victims should be aided, but the punishment should not be death. Due to our flawed and dysfunctional criminal justice system, there is a great chance that innocent people may become victims of wrongful convictions."

Fr Secillano and opposition lawmakers also criticised the tactics used to ensure the bill was passed, such as curtailing of debates and only allowing a vote by voice so politicians would not be specifically identified as having supported it.

The speaker of the house also threatened to strip politicians of committee leadership positions if they voted against the bill.

"This is a chamber of puppets and bullies," congressman Edcel Lagman, a longtime opponent of capital punishment, said after his efforts to block the bill were voted down.

Bill limits the death penalty to drug-related crimes

Possessing 500g of marijuana, or 10g of cocaine, heroin or ecstasy, would be crimes punishable by execution, as would manufacturing and selling drugs.

People who commit serious crimes such as murder and rape while under the influence of drugs could also be executed.

However committing those crimes without being under the influence of drugs would only be punishable with jail terms.

The bill allows for execution by hanging, firing squad or injection.

Mr Duterte won presidential elections last year after pledging an unprecedented campaign to eradicate illegal drugs in society by killing tens of thousands of people.

Since he took office in May, police have reported killing more than 2,550 people in the drug crackdown, claiming all the deaths were in self defence, while more than 4,000 others have died in unexplained circumstances.

Rights groups and other critics have said Mr Duterte is presiding over widespread human rights violations, with Amnesty International warning the killings could amount to a crime against humanity.

Many Filipinos support Mr Duterte and his drug war, arguing extreme measures must be taken to halt crime.

The United Nations had warned bringing back the death penalty would violate international conventions the Philippines had already ratified.

After capital punishment, another priority bill for Mr Duterte is a companion bill lowering the age of criminal liability to as low as nine-years-old, from 15 currently.