Italian prosecutors are investigating the possible poisoning death of a Moroccan model who was a key witness in the trial of former prime minister Silvio Berlusconi over his infamous "bunga bunga" parties.

Key points: Imane Fadil testified against former premier Silvio Berlusconi

Imane Fadil testified against former premier Silvio Berlusconi He was accused of paying for sex with an underage prostitute, but acquitted

He was accused of paying for sex with an underage prostitute, but acquitted Toxicological tests on Ms Fadil indicated a "mix of radioactive substances", according to a local report

Imane Fadil died on March 1 at a Milan hospital, where she had been treated since January 29 exhibiting "symptoms of poisoning", Milan chief prosecutor Francesco Greco said, according to Italian news agency ANSA.

But her death was only announced on Friday, more than two weeks after she died.

"The doctors have not identified with any certainty any pathology which can explain the death," Mr Greco said, adding there were "several anomalies" in her medical records.

Ms Fadil reportedly told her friends and lawyer she had been poisoned.

And in 2012, Ms Fadil had told reporters she feared for her safety after telling prosecutors investigating possible witness tampering in the case that she was offered money in exchange for her silence about what went on at Mr Berlusconi's parties.

In a statement reported by ANSA, the Humanitas hospital said it provided the results of toxicological exams to prosecutors when they were completed on March 6.

ANSA quoted unnamed officials as saying the tests indicated a "mix of radioactive substances". Autopsy results are pending.

Silvio Berlusconi was acquitted in 2015 of charges he paid for sex with an underage woman. ( AP: Andrew Medichini )

Ms Fadil told court of 'bunga bunga' parties

Ms Fadil testified at the 2012 trial of Mr Berlusconi, who was accused of paying for sex with an underage prostitute.

She told the court one of the parties at the media magnate's home involved young women, sometimes in pairs, wearing nun's costumes and stripping off while performing raunchy pole dances.

"They started to dance like the nuns of the film Sister Act, and then they took off their clothes," she said.

Mr Berlusconi was initially convicted in the case but ultimately acquitted in 2015 after a judge ruled he could not have known the underage prostitute was a minor.

However, magistrates subsequently laid new charges against Mr Berlusconi and other defendants, accusing them of bribing some of the women who attended the parties to keep them from telling the truth at the initial trial.

They have denied the accusations.

AP/Reuters