"The accused planned suicide car bombings outside the embassies of France and the United States in Egypt," MENA said
In the case of the French mission, the motive was to register a protest "against French military intervention in Mali," investigators were quoted as saying.
The news agency did not say why the American embassy was to be targeted.
On Saturday, Egyptian Interior Minister Mohamed Ibrahim said police had arrested three members of a "terror cell" involved in a transnational plot to bomb a Western embassy and other targets.
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MENA said the three were Egyptian and quoted security sources as saying they had been among prisoners who managed to escape from prison during the 2011 uprising against former president Hosni Mubarak.
Two of the suspects had been extradited in 2009, one from Algeria and the other from Iran, the agency said.
The three admitted having been in contact with al-Qaeda but denied plotting any attacks, the agency added.
On Saturday, Ibrahim said they had been in touch with Kurdi Dawud al-Assadi who is "the head of al-Qaeda in some west Asian countries."
The suspects' lawyer said he attended their questioning on Saturday and claimed the prosecution had no evidence, including disputing that they were caught with explosives.
"This is just a case of the security apparatus trying to assert its worth," Mamduh Ismail told AFP.
MENA said on Saturday prosecutors had ordered two of the suspects, from the Mediterranean city of Alexandria, detained for questioning for a renewable 15-day period, and that the third was under house arrest.
Two of the suspects had also travelled to Mali, where French troops battled al-Qaeda-inspired militants, MENA reported.