A Philippine senator has warned that an impeachment process could be initiated against President Rodrigo Duterte after he admitted having killed alleged drug addicts when he was mayor of Davao city.

"When he says that, he's opening himself up, isn't he? He said it, so the legal way is to go ahead and impeach him," Senator Richard Gordon said in reference to Duterte's remarks.

"I'm sure he knows how to defend that," added Gordon, an independent politician, who normally supports the president's policies.

In a speech on Monday before departing on an official visit to Cambodia, Duterte declared he had killed alleged drug addicts and traffickers while he was mayor of Davao city.

Since Duterte was elected president in May, the war on drugs by his administration has killed closed to 6,000 people, out of which over 2,000 have died in police operations and the rest in extrajudicial killings.

The violent campaign against drug-trafficking has been strongly criticised by international and regional human rights groups, including the United Nations and the European Union.