Jury members in Bill Cosby's sex assault trial are going back to their hotel for the night after failing to resolve a deadlock that's threatened to end the case without a verdict.

Jurors worked into the night Thursday after Judge Steven O'Neill told them to try and resolve the impasse. They've been deliberating for nearly 40 hours over four days.

They'll resume deliberations Friday morning in suburban Philadelphia.

As deliberations continued on the fourth day, nerves were frayed and patience was shot.

Some jurors appeared angry, the judge sounded exasperated and Canadian accuser Andrea Constand's mother broke down in tears Wednesday.

The sequestered jury has been working late into the night since getting the case Monday, pausing a half-dozen times to revisit key evidence, including Cosby's decade-old admissions that he fondled Constand after giving her pills.

O'Neill seemed vexed at times as the court staff struggled to answer the jury's requests. One batch of requested testimony hadn't even been transcribed yet.

'This is an incredible jury'

But when jurors asked to stop for the day Wednesday night, O'Neill was effusive with praise — encouraging their diligence as they weigh charges that could put the Cosby in prison for the rest of his life.

"This is an incredible jury that has just acted with incredible dignity and fidelity," O'Neill said. "I don't have any higher praise. You have taken your task so seriously."

Cosby is charged with three counts of aggravated indecent assault. Each carries a maximum 10-year prison term, though the counts could be merged at sentencing if he is convicted.

The case has already helped demolish his nice-guy reputation as America's Dad.

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Cosby arrives Wednesday morning for the start of Day 3 of jury deliberations in his sexual assault trial. (Brendan McDermid/Reuters)

Cosby has wavered between stoic and smiling as he awaits his fate, but gave a brief thumbs-up as jurors listened to a court reporter reread his January 2005 police interview.

In it, he claimed Constand showed no ill effects from the 1 1/2 Benadryl pills he gave her to help her relax, and that she never objected to his behaviour during the 2004 encounter at his suburban Philadelphia home.

Constand testified last week that she was paralyzed by the pills and unable to fight Cosby off. Her mother, Gianna Constand, pulled a cloth from her pocket to wipe away tears Wednesday as she listened to the testimony.

Cosby's lawyers maintain Constand was a willing sexual partner.

Bill Cosby

Complainant Andrea Constand is seen arriving to testify at the trial last week. (Mark Makela/Associated Press)

Some jurors closed their eyes and tilted their heads down as they listened to the police interview. One slunk down in his seat, looking angry.

"Can you find 12 people who will agree? That's the question," said criminal lawyer Alan J. Tauber, who wasn't involved in the case.

The Associated Press does not typically identify people who say they are victims of sexual assault unless they grant permission, which Constand has done.