The Egyptian judges trying dozens of democracy activists, including Americans, in a case that has strained ties with Washington recused themselves from the trial Tuesday, judicial sources said.
In such instances, the trial, which opened on Sunday, is normally referred to the court of appeals, which appoints a new panel of judges.
State news agency MENA reported that chief judge Mohammed Shukry sent a letter to the head of the appeals court, which designates trial judges, saying that he and his two colleagues could not continue the trial.
It quoted them as using a formulation that could either mean they felt unease at the proceedings or restrictions on their work.
Hafez Abu Saada, a lawyer for three of the defendants, said judges often recuse themselves over a conflict of interest or a sense of pressure.
"Maybe one of the parties in the trial has a relation with one of the judges, or he felt he was facing pressure, whether from the media or public pressure, which won't allow him to judge fairly," he told AFP.
He said it was too early to tell whether to welcome the move, which could lengthen the trial.
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But Egyptian media have reported that the authorities were considering a way to resolve the rift with Washington, perhaps by retroactively licensing the civil society groups.
The trial opened amid continuing US pressure on its close ally to drop the charges against the activists, who worked with five foreign NGOs accused of receiving illicit foreign funds and operating without licenses.
Earlier on Tuesday, US Secretary of State Hillary Clinton told senators in Washington that the administration was "engaged in very intensive discussions" with Egypt and hoped "to resolve this very soon."
Egyptian judicial sources say 19 Americans are among the 43 defendants of various nationalities. None of the foreigners showed up at Sunday's hearing.
Several of the Americans have sought refuge in their Cairo embassy, including Sam LaHood, the head of the US-based International Republican Institute and son of US transportation secretary Ray LaHood.
The other defendants are Egyptian, Palestinian, Norwegian, Serbian and German. Four of the groups are American and the remaining one is German.
The trial sparked a rift between Washington and Egypt's military rulers, who took power after an uprising ousted president Hosni Mubarak a year ago.
US legislators have threatened that pressing on with the trial could imperil more than $1.3 billion the US yearly gives to Egypt, mostly to the military.