"Not funny. Ever," Chelsea Clinton, the daughter of former US president Bill Clinton, wrote on Twitter.

In a second post Ms Clinton wrote: "Duterte is a murderous thug with no regard for human rights. It's important to keep pointing that out & that rape is never a joke."

In a speech at a military camp, Mr Duterte told soldiers they have his full support to implement the crackdown that gives them authority to detain anyone or go anywhere, and only he would be responsible.

"I will go to jail for you. If you happen to have raped three women, I will own up to it," he said.

Phelim Kine of Human Rights Watch said the comment was a "sickening attempt at humour" that sent soldiers a signal they could commit human rights abuses while enforcing martial law.

The Philippine women's party Gabriela said: "Rape is not a joke. Martial law and the heightened vulnerability to military abuse that it brings to women and children are not a joke either."

Before taking office last year, Mr Duterte caused uproar when he joked about raping Australian missionary Jacqueline Hamill when prisoners seized control of a jail in southern Davao City in 1989.

He said as mayor of the city at the time he should have been "first in line".

Responding to condemnation over those remarks, Mr Duterte said: "I do not want anybody controlling my mouth. I say what I say and I've said it and if it does not sit well with you, that's your problem."

Mr Duterte is notorious for making crass and outrageous comments, forcing his aides to explain away his most controversial ones as "merely rhetoric" or an "exaggeration".

His spokesman Ernesto Abella released a statement at the weekend saying that in his latest remarks the president was using "heightened bravado" when trying to raise the morale of troops.

Mr Duterte, who likes to be known as "The Punisher", has in the past called US president Barack Obama the "son of a whore", said he will kill as many drug addicts as Hitler did Jews and vowed to kill 100,000 criminals.

He also said fish in Manila Bay would grow fat from eating drug addicts and pushers under his so-called war on drugs.

More than 7000 mostly poor Filipino drugs suspects have been subsequently killed in what human rights activists say could amount to crimes against humanity.

Mr Duterte last week imposed martial law across Mindanao, the country's second-largest island and home to 20 million people, after militants linked to Islamic State rampaged through the city of Marawi, 830 kilometres south of Manila, torching key government buildings and a church and taking hostages.

Fighting in the city where most of 200,000 residents have fled continued on Saturday, the first day of the Islamic fasting month of Ramadan, as militants remained holed up in buildings for a fifth day.

Military helicopters fired guided rockets at militant positions.

Up to 50 people have been killed and dozens wounded in the city since last Tuesday.