The father of jailed Al Jazeera journalist Peter Greste has said his imprisonment in Egypt is a "grave and very serious injustice", days after the family announced an appeal.

Juris Greste on Thursday said he accepted the integrity and independence of Egypt's judicial system, but passionately believed in the innocence of his son.

Greste, along with two other Al Jazeera journalists, Mohamed Fahmy and and Baher Mohamed, was jailed for seven years on charges of aiding the Muslim Brotherhood, a banned group, and reporting "false news" in the aftermath of a coup against the Brotherhood-backed president, Mohamed Morsi.

At a news conference in Kenya, Greste's 78-year-old father said: "We accept the integrity and independence of any country's judicial system and administration of law; in no way would we want to challenge that", he said. 

"However, we passionately and absolutely know that a grave and very serious injustice has taken place in Peter's case and we won't shy away from professing that position as long as we can."

Baher Mohamed was sentenced to a further three years in prison for "possessing ammunition". The evidence against him was a spent bullet casing he had picked up during a protest in Cairo.

Evidence against the journalists included a pop video, Greste's holiday photos from Germany, a BBC report about Somalia and a Sky News Arabia film about an Egyptian horse sanctuary. 

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Greste is "coping well" in jail since his conviction by an Egyptian court, his parents said, but said the family were devastated by the many months he has already spent behind bars.

"In his mind, and in ours too, there's absolutely no way he's going to be there for the next seven years," his mother, 75-year-old Lois Greste, said. 

Last week, Greste's family said he would appeal. 

"It's a matter of going through the appeals process, and hopefully we will get to the stage where they realize that this is a mistake and free him," she added, noting she was talking about all "three boys."

In a statement released through his family, Fahmy said he was still deciding whether or not to appeal against his sentence. 

"I have not taken a final decision yet regarding appealing the unacceptable draconian verdict against me, mainly because I have no faith in the judicial system", he wrote.

"If an appeal is accepted months from now, we will be paraded in whites again in the circus of a retrial.

"We are innocent and the world may be understandably too busy with the wars in Gaza, Israel, Iraq, Syria, and Ukraine to realise ... I am hopeful that a breakthrough may happen sooner rather than later."

The case sparked a global outcry and demands for a presidential pardon amid claims it was a politically motivated trial.

Abdel Fattah el-Sisi, Egypt's president, has said he cannot consider a plea of clemency or a pardon until all legal proceedings have been concluded, and that includes an appeal.

Al Jazeera rejects the charges against its journalists and maintains their innocence.

Source: Al Jazeera and agencies