Egypt’s acting president has written to the family of jailed Australian journalist Peter Greste, promising to do all he can to resolve the case quickly.

Egypt’s interim president Adly Mansour says he “will spare no effort” to quickly resolve the case of jailed Australian journalist Peter Greste.

Mr Mansour, who is also head of the country’s constitutional court, has written a letter in response to one sent by the Al-Jazeera journalist’s parents Lois and Juris Greste.

Greste is facing up to seven years in jail, accused of backing the black-listed Muslim Brotherhood and portraying Egypt in a state of “civil war”.

His mother read out the interim president’s letter at a press conference in Brisbane on Tuesday.

“As a father I’d love to convey my understanding of the prevailing emotions of distress and anguish amongst both Peter and the whole family,” he wrote.

“Notwithstanding the independence of the judiciary authority and foremost all the rights guaranteed by the law, I would like to assure you in my capacity as president of Egypt that I will spare no effort to work towards the speedy resolution of the case in a fashion consistent with the law and that guarantees the resumption of the family in the near future.”

The Greste family believes there is little or no evidence to substantiate the charges against the journalist, who is due to reappear in an Egyptian court on March 24.

Al-Jazeera has been leading an international campaign demanding the release of Greste and eight other network staff, and has denied all charges against them.

Andrew Greste, who returned to Australia from Cairo on Saturday, said his brother was being held in “extremely tough” conditions.

He was in a dark, three by four metre cell with two others journalists. Until last week they had been denied reading material, and they are still not allowed to have writing implements.

Andrew Greste said his brother was doing his best to stay strong, and not to dwell on the prospect of a jail term.

“Those things … they’re out of his control and they can become quite soul destroying,” he said.

Greste’s father Juris said prison guards had recently removed a sign Peter and his cell mates had made that read: Freedom Now.

His mother said Greste had already replaced it with a round disc of foil, which catches the light when the sun hits it.

“He sees that as light coming into his life,” she said.

“They’re doing amazing creative things to buoy themselves and keep their spirits up.”

Australian Greens leader Christine Milne said it was clear the Egyptian authorities were under pressure from the international community, and it was now time for the Abbott government to step up.

“I would call on the Australian government to absolutely redouble efforts to get Peter Greste home,” she told reporters in Canberra on Tuesday.