Egypt's national defence council has warned that reconciliation talks with supporters of deposed president Mohamed Morsi will not be open-ended, suggesting that security forces will eventually break up the ongoing sit-ins across the capital.

The US, EU and Gulf states are trying to negotiate a deal to end the Muslim Brotherhood-led protests, which have continued for more than five weeks.

William Burns, the US deputy secretary of state, extended a visit to Cairo by an extra day on Sunday to meet defence minister Abdel Fattah el-Sisi and other officials.

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But as the meetings continued, the council warned that talks would be "defined and limited and ... not infringe on the law and rights of citizens," and that "law-breakers" would still face punishment.

It also called on the protesters to abandon the sit-ins and join the political road map announced the day of Morsi's overthrow. The statement was the latest warning to thousands of protesters camped out outside a mosque in Cairo's Nasr City district, and near Cairo University in Giza.

At the core of the negotiations is the political future of the Brotherhood. The group says it is looking for concessions before beginning talks with the new military administration, which could include releasing detained Brotherhood leaders, unfreezing the group's assets, and lifting a ban on Islamist TV channels which were shut down last month.

Also on Sunday, the Cairo appeals court announced an August 25 date for the trial of six Brotherhood officials on charges of murder and incitement.

The defendants include the group's leader, Mohamed Badie, and its leading strategist, Khairat al-Shater, who was arrested last month.

The group is accused of inciting clashes outside their headquarters on June 30, which left at least 12 people dead.

Prosecutors also ordered Morsi's top aide, Rifaa Tehtawi, held for 15 days pending an investigation into charges connected to last December's clashes at the presidential palace. Tehtawi was detained last month along with the former president.

Source: Al Jazeera and agencies