MANILA (REUTERS) - Philippine President Rodrigo Duterte's allies in Congress on Tuesday (Dec 13) filed a criminal complaint against a senator who has been an outspoken critic of a surge of extrajudicial killings unleashed by the President's campaign against drugs.

The legal complaint against Senator Leila de Lima, who was justice minister in a previous administration, is the latest government action against her since she led an inquiry into the killings.

She has denounced the government attacks on her as "madness"and "harassment" and has appealed to Mr Duterte to stop the string of bizarre accusations and insults.

Senators loyal to Mr Duterte ousted her from the leadership of the investigation in September.

Congressmen then accused her of disrespect in connection with criticism she made of her treatment and the fate of the inquiry. That led to a criminal complaint.

"Senator de Lima has shown disrespect to the House of Representatives," said Congressman Reynaldo Umali, head of the lower house's justice panel. "We cannot allow this incident to pass, how can we earn the respect of the people when a senator insults us by calling the inquiry a sham and a kangaroo court."

Ms de Lima is on an official trip to the United States and Europe and was not available for comment.

Mr Duterte's war on drugs, the key plank of his campaign for a May election, has claimed about 5,000 lives since July 1.

The high toll and mysterious circumstances of some of the killings have alarmed rights groups, the United States and United Nations, whose concerns have drawn angry rebukes from Mr Duterte.

Mr Umali said Ms de Lima faced imprisonment of up to six months if found guilty.

A small group of opposition politicians in Congress said the case against Ms de Lima would have a chilling effect on critics of Mr Duterte's war on drugs.