The future of three al-Jazeera journalists jailed in Egypt looks bleaker after Egypt's strongman president, Abdel Fatah al-Sisi, refused a pardon, ignoring pleas from Barack Obama to release them and other political prisoners.

"We will not interfere in judicial rulings," Sisi said on Tuesday morning. "We must respect judicial rulings and not criticise them even if others do not understand this."

Australian Peter Greste, Canadian-Egyptian Mohamed Fahmy and Egyptian Baher Mohamed were jailed for between seven and 10 years on Monday for endangering Egypt's national security, alongside four students and activists. Two British al-Jazeera journalists and a Dutch freelancer were sentenced to a decade in jail in absentia – despite the prosecution, according to trial observers Amnesty International, failing "to produce a single shred of solid evidence".

Sisi's refusal to intervene comes just under a year after he ousted Mohamed Morsi in what was framed at the time as an attempt to preserve Egypt's democracy. It was a further slight to US diplomacy, coming just hours after the White House demanded the journalists' release, and only two days after America's top diplomat, John Kerry, reiterated that millions of dollars in suspended aid money to Egypt would be unfrozen.

"We call on the Egyptian government to pardon these individuals or commute their sentences so they can be released immediately, and grant clemency for all politically motivated sentences – starting with the other defendants in this trial," the White House said in a statement.

"The prosecution of journalists for reporting information that does not coincide with the government of Egypt's narrative flouts the most basic standards of media freedom and represents a blow to democratic progress in Egypt."

Sisi's failure to intervene means that a lengthy appeals process – which may not begin until October – is the only legal recourse left available to Greste and his colleagues. On Tuesday morning, Greste's brother Mike said the family were still mulling over options, and would delay announcing their next action until meeting with legal experts and lawyers on Tuesday and Wednesday. "It's too early to say," said Mike Greste.

While Sisi would placate international critics with a pardon, inside Egypt he would gain little, with many applauding the convictions. Al-Jazeera has been portrayed as an enemy of the state due to the perception that its coverage favours the supporters of Egypt's ousted president, Mohamed Morsi.

The defendants in the trial have been portrayed as terrorists – known in the media as "the Marriott cell" – and students with Islamist links were added to the case to make it seem like the journalists had been collaborating with dissidents. In a televised broadside on Monday night that embodied many Egyptians' anti-Jazeera sentiment, the TV presenter and newspaper editor Ibrahim Eissa applauded the jailing of Greste and his colleagues.

"None of them is a journalist, [none of them is] a journalist who is a member of the Egyptian journalists' syndicate, or a journalist who is working in Egyptian journalism," said Eissa, who ironically was a fierce regime critic under Hosni Mubarak, and who was once himself pardoned by Mubarak after being sent to jail for his journalism. "There is no one like that among them. All of them are students and all of them are [Muslim] Brotherhood."

Eissa's outburst followed yet more blows to free expression in Egypt. In Minya, central Egypt, a reporter was jailed for five years on Monday for reporting on sectarian attacks on Christians in the area. Mohamed Hegazy, who works for a US-based channel, was reportedly accused of fabricating news and exacerbating sectarian tensions.

Later, Egypt's best-known contemporary novelist, Alaa al-Aswany, said he would no longer write columns in the country's most-respected private broadsheet, al-Masry al-Youm, because "criticism and difference of opinion is no longer allowed". Aswany's statement was striking because he was initially one of the loudest cheerleaders of last summer's regime change – although his joy has since turned to criticism of the current regime.

Egypt's attack on free expression is part of a wider crackdown on dissent that has seen hundreds "disappeared", and at least 16,000 arrested, according to the government's own figures – with some estimates rising to 41,000.