Philippine lawmakers allied with President Rodrigo Duterte on Tuesday voted to allocate an annual budget of just 1,000 pesos ($19.66) to the Commission on Human Rights, a public body investigating Duterte's bloody war on drugs.

About 119 of the 151 lower house members present supported the move to dramatically cut the budget, in what critics of the anti-drugs campaign call retaliation for the agency's criticism of Duterte and its efforts to investigate thousands of killings over the past 15 months.

The rights agency deserved a low budget for being a "useless" body and defending criminals' rights, the speaker of the house of representatives, Pantaleon Alvarez, said in a television interview.

"If you want to protect the rights of criminals, get your budget from the criminals," he said. "It's that simple. Why should you get budget from the government and yet you are not doing your job?"

Thirty-two minority lawmakers opposed the measure, said Congressman Edcel Lagman, adding that the president's supporters were "virtually imposing the death penalty on a constitutionally created and mandated independent office".

The agency requested a budget of 1.72 billion pesos for 2018, but the government proposed 678 million pesos.

On the second reading of the legislation, Congress approved that the figure be slashed to just 1,000 pesos, a huge cut from the 2017 budget of 749 million.

Though the motion still requires another reading and Senate approval, opponents say it is likely to be passed, as Duterte enjoys a supermajority in the two chambers.

The agency has long complained it lacks manpower and resources to fully investigate the killings.

Its head, Chito Gascon, said the measly budget was an attempt to force his resignation.

"The principal reason why I cannot resign my office is that to do so is to weaken the institution itself," Gascon said.

He added: "On pretext, asking me to resign would lead to essentially making the institution forever at the mercy of politics."

Filipinos are largely supportive of Duterte's crackdown as a solution to tackling rampant crime stemming from drug addiction.

However, activists say the prime targets have been users and small-time peddlers and accuse police of executing thousands of people. The police reject that and say they kill only in self-defence.

Human rights advocates hope senators will restore the agency's current budget.

"If we need to go to the Supreme Court on this issue we will consider that as well," added Gascon.

Lawmakers may have misunderstood the agency's role, said one representative, Raul del Mar.

"We need to be reminded that the CHR's main function is to curb the excesses and abuses of those in the seat of power," he said.