He did not identify the embassy, but said the militants "were on the verge" of attacking it using a suicide bomber or by detonating a bomb packed with ammonium nitrate -- a common fertiliser.
Ibrahim said the suspects were captured with 10 kilos (22 pounds) of the fertiliser, and a computer containing instructions on bomb-making.
The militants had been in touch with an Al-Qaeda leader outside the country, identified as Kurdi Dawud al-Assadi who is "the head of Al-Qaeda in some west Asian countries," Ibrahim said.
One of the suspects associated with Al-Qaeda members in Algeria and also received training from the loose-knit militant organisation in Pakistan and Iran, Ibrahim said.
"They were in electronic communication with Al-Qaeda in Pakistan," he said, adding that they were also in touch with an Al-Qaeda facilitator on the Turkish border.
He did not specify which of the eight countries that Turkey shares a border with, although these include Iran, Iraq and Syria.
The suspects' lawyer, an Islamist who has defended other suspected militants in the past, said he attended their questioning on Saturday and claimed the prosecution had no evidence.
"There was no evidence, nothing," Mamduh Ismail told AFP. "This is just a case of the security apparatus trying to assert its worth," he said. He disputed the allegation that the suspects were caught with explosives.
The official MENA news agency reported that prosecutors have ordered two of the suspects, from the Mediterranean city of Alexandria, detained for questioning for a renewable 15-day period, and said the third is under house arrest.
MENA reported a conflicting account from the prosecutors' investigation which alleged the first two suspects had planned to bomb "a number of Western embassies."
Two of the suspects had also travelled to Mali, where French troops battled Al-Qaeda-inspired militants, MENA reported.
According to Ibrahim, Assadi had instructed the suspects to coordinate with two alleged militants before their capture last October after a firefight that killed a gunman in a Cairo apartment.
The suspects arrested in October, also alleged to have Al-Qaeda links, are now on trial.
Fuad Allam, a retired interior ministry general who helped crack down on Islamist militants in the 1980s, said he believed militants identifying with Al-Qaeda have begun to coalesce after the Arab Spring uprisings.
"The terror cells (in the region) are beginning to gather, they seem to have an organisation," he told AFP.
One of the suspects arrested in October allegedly had links to an attack last September 11 on the US consulate in the Libyan city of Benghazi that killed ambassador Chris Stevens.
Egypt has in the past announced the arrest of Al-Qaeda-linked militants in the country which has seen a low-level Islamist insurgency and militant attacks on tourist sites over the past three decades.
Some veteran Egyptian Islamist militants are now in Al-Qaeda's leadership -- most prominently Ayman al-Zawahiri who heads the extremist organisation founded by Osama bin Laden.
Others had been jailed and recanted violence before their release following the 2011 uprising that overthrew president Hosni Mubarak.
There has been a resurgence in militant attacks since the uprising, confined to the sparsely populated Sinai peninsula, in which both Egyptian troops and neighbouring Israel have been targeted.