Egyptian political movement the Muslim Brotherhood says it will reject the offer of cabinet posts in any new transitional government formed by Prime Minister Hazem el-Beblawi.
The apparent gesture of reconciliation was one of the first acts by Mr el-Beblawi after his appointment on Tuesday, but the Brotherhood said it would settle for nothing less than the return to the presidency of Mohammed Morsi.
Mr Morsi, an Islamist and Egypt's first freely elected leader, was removed from office by the army last week after mass protests. His supporters accuse the military of staging a coup, but opponents say the move is the continuation of the revolution that deposed President Hosni Mubarak in 2011.
Senior officials from the Brotherhood told the BBC that it would play no part in the future government while Mr Morsi was still being detained and only two days after 51 of his supporters were shot dead in clashes outside the military headquarters in Cairo on Monday.
The appointment of Mr el-Beblawi and other interim officials ahead of proposed elections in 2014 is the first serious attempt by the military to move on from last week's events, despite the turmoil and divisions across Egypt.
But those plans already seem to be in disarray after the main liberal opposition coalition also rejected the new poll timetable to stop the unrest. The National Salvation Front (NSF) has demanded more changes and consultation.
The Muslim Brotherhood and the Tamarod protest movement earlier rejected the decree announced by interim president Adly Mansour on Tuesday. The Mansour plan also envisages changes to the Islamist-drafted constitution.
Mr Mansour's office has also announced that the liberal politician and Nobel Peace Prize laureate, Mohamed ElBaradei, should become deputy president with responsibility for foreign affairs. The announcement is aimed at trying to restore some semblance of stability to Egypt.
The choice of Mr Beblawi immediately won the acceptance of the ultra-orthodox Islamist Nour Party, a sometime ally of the toppled president Mohammed Morsi and his Muslim Brotherhood.
Nour leaders have been courted by the military backed interim authorities to prove that Islamists will not be marginalised by the new government. Mr el-Beblawi did not originally feature as a likely candidate, but a BBC correspondent says the fact he has experience as a former finance minister sends a message that the country's failing finances will be a major priority.
Meanwhile, two people have been killed after suspected Islamists attacked a security checkpoint in the Sinai province on Wednesday. Reports say six people were injured in the attack near Egypt's border with Israel. The violence comes three days after an attack on a gas pipeline in Sinai.
