CAIRO (AFP) - Egypt's prosecutor is investigating complaints of "threatening public security" against popular satirist Bassem Yousef, who is out on bail facing accusations of insulting President Mohamed Morsi and Islam.

Judicial sources and Mr Youssef himself said the public prosecutor ordered the investigation on Monday.

"A new complaint against me has been referred to state security prosecution, for spreading rumours and false news, and disturbing public tranquility after the last episode," Mr Yousef wrote on Twitter.

"It seems they want to drain us physically, emotionally and financially," he added.

Judicial sources said a lawyer had lodged a complaint against Mr Youssef and the head of the CBC television channel which airs his weekly programme Albernameg (The Show), which is modelled on Jon Stewart's satirical The Daily Show.

The complaint appears to accuse Mr Youssef of stoking criticism of Islamists and obliquely calling for a "civil war".

Mr Youssef, who regularly skewers the country's ruling Islamists on his wildly popular show, was released on US$2,200 (S$2,730) bail on Sunday after an interrogation that lasted nearly five hours.

He was questioned on accusations of offending Islam through "making fun of the prayer ritual" and of insulting Mr Morsi by "making fun of his international standing". He now joins the ranks of several colleagues in the media who face charges of insulting the president.

The soaring number of legal complaints against journalists has cast doubt on Mr Morsi's commitment to freedom of expression - a key demand of the popular uprising that toppled former leader Hosni Mubarak in 2011.

The United States on Monday expressed concern at the proceedings against Mr Youssef, saying it was evidence of a "disturbing trend" of mounting restrictions on freedom of expression.

"We are concerned that the public prosecutor appears to have questioned and then released on bail Bassem Youssef on charges of insulting Islam and President Morsi," US State Department spokeswoman Victoria Nuland said in Washington.

"This, coupled with recent arrest warrants issued for other political activists, is evidence of a disturbing trend of growing restrictions on the freedom of expression."

Under Egypt's legal system, complaints are filed to the public prosecutor, who decides whether there is enough evidence to refer the case to trial.

Suspects can be detained during this stage of investigation.

Rights lawyers say there have been four times as many lawsuits for insulting the president under Mr Morsi than during the entire 30 years that Mr Mubarak ruled.