The case of slain Italian student Giulio Regeni has become "very difficult" because of the "hostile campaigns" that consistently raise doubts about the Egyptian interior ministry's efforts in handling the case, said interior minister Magdi Abdel-Ghaffar on Sunday.

“These campaigns are primarily launched by the media, which is raising doubts all the time about the interior ministry's efforts in exposing the truth about Regeni's murder,” Abdel-Ghaffar told reporters after attending Prime Minister Sherif Ismail's policy statement before parliament.

The interior ministry faced a wave of criticism after saying on Thursday that the police had found identity documents belonging to Regeni in an apartment linked to what the ministry says was a gang that robbed foreigners, hours after police killed all four of its alleged members in a shootout.

Critics have questioned the plausibility of robbers who target foreigners having a motive to torture Regeni for days and holding onto his potentially incriminating passport and identity documents.

Investigations into the murder are still underway.

"There is full cooperation between Egypt's interior ministry and Italy's security apparatuses," Abdel-Ghaffar said.

"We are exchanging information on this case all the time and there is an Italian security delegation here in Cairo tasked with following the investigation step by step.

"Cooperation with the Italian side is natural because the case is very difficult and shrouded in mystery from all sides."

Italy's interior minister Angelino Alfano said on Sunday that Egyptian investigators in the case agreed to extend the investigation after pressure from Rome, AFP reported.

Regeni's body was found with signs of torture on a roadside in Giza nine days after he disappeared on 25 January.

The 28-year-old PhD student was conducting research on independent trade union movements in Egypt.