The United States on Monday denounced "new harsh sentences" handed down by an Egyptian court on opposition activists and urged the defendants to appeal their jail terms.

Alaa Abdel Fattah, a leading dissident in the 2011 uprising that toppled strongman Hosni Mubarak, was jailed for five years over an illegal protest.

The remaining 24 defendants in the case received sentences ranging from three to 15 years.

"As a matter of principle, the United States believes that a country's long-term stability is strengthened by protecting the right of its citizens to peacefully express dissent," State Department spokeswoman Jen Psaki told reporters.

"These sentences and others under the law have had a chilling effect on key freedoms of expression and assembly."

Psaki called on Cairo "to quickly complete its review of the demonstrations law and all court verdicts issued under it."

And she said Washington would "urge defendants to pursue all legal avenues to contest this verdict, including the right to appeal."

The US administration has been caught in a quandary about how to deal with Cairo, and the new government led by former general and now President Abdel Fattah al-Sisi.

A tranche of the $1.5 billion in annual US aid to Egypt has been frozen as Washington pushes Cairo to ensure that the country is on a path to democracy after the ousting of Mubarak.

The veteran leader was replaced by the Egypt's first democratically elected president, Mohamad Morsi, but the Islamist leader was toppled after a year by the military, which then oversaw Sisi's election.