Egypt's former top auditor Hisham Genena has been ordered detained for 15 days for questioning over his claims that the country's former army chief Sami Anan was in possession of "documents and evidence" that would undermine the country and its leadership.

Genena's lawyer confirmed to local media on Tuesday he was taken from his home in the New Cairo district of the capital.

"The military prosecution has decided to detain the former head of the Central Auditing Organization, Judge Hisham Genena, for 15 days for investigation," the official news agency MENA said.

The former anti-corruption chief had been assisting Anan in his presidential campaign before it was abruptly suspended last month.

His arrest comes after the Supreme Committee of the Armed Forces (SCAF) said it would launch an investigation into both the ex-presidential candidate and his campaign manager over remarks in which Genena claimed Anan held secret "documents and evidence" that would incriminate people in power for incidents that followed the January 25 uprising.

In an interview with HuffPost Arabi on Sunday, Genena said Anan threatened to leak those documents if he were to be legally charged.

He also expressed concern over Anan's safety, saying he feared a potential assassination attempt.

US calls for credible elections

Egyptians will go to the polls next month in a presidential election in which incumbent Abdel Fattah el-Sisi looks poised to extend his term in office.

Several potential candidates, like Anan, have either been arrested or faced threats, intimidation and physical violence, forcing them to drop out.

Amid the opposition crackdown, the US Department of State called for "free and fair elections".

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Spokeswoman Heather Nauert told reporters on Tuesday that Secretary of State Rex Tillerson raised the issue of human rights, recent arrests and elections with Sisi during his recent visit to Cairo.

Regarding Genena's detention and probe, she said: "We're following that case closely. We're certainly aware of that."

"We support a transparent and credible process with regard to the electoral process, and that's one of the reasons why the secretary spoke about the importance of having full, unfettered access to the polls, so that people can vote," Nauert added.

Anan, who is held in a military prison, was detained last month on allegations he committed violations that "warrant official investigation", and incitement against the army.

Before his arrest, Genena was attacked on January 27 during a suspected kidnapping attempt, his lawyers said.