A bomb near an army intelligence building wounded four soldiers Sunday, the third such blast within a week after the Muslim Brotherhood's designation as a terrorist organisation further polarised Egypt.
The explosion, which the army called a "cowardly terrorist" act, comes as the military-installed authorities plan to hold a referendum on a new constitution next month, the first step towards democracy since the ouster of Islamist president Mohamed Morsi in July.
Sunday's blast in Sharqiya province in the Nile Delta destroyed the rear compound wall of the intelligence building, the army said, adding that four soldiers were wounded.
It was the third such attack in less than a week.
Another was averted on Sunday when experts defused a bomb near the front gate of Al-Azhar's medical faculty in New Damietta city north of Cairo, security officials said.
On Tuesday, a suicide car bomber killed 15 people at a police building in Mansoura, north of the capital. And on Thursday, a bomb in Cairo wounded five people on a bus.
The Mansoura attack, one of the deadliest since Morsi's ouster, triggered widespread outrage.
A day after that attack, which was claimed by Al-Qaeda-inspired jihadists, the authorities accused the Brotherhood of perpetrating it and listed the Islamist movement to which Morsi belongs as a "terrorist organisation".
But the Brotherhood, which prevailed in all elections since the ouster of strongman Hosni Mubarak in early 2011, said it is "innocent of any violent incident that has (been) or will be committed".
Designating the Brotherhood as a terrorist group is a hardening of the government position in its ongoing crackdown on Morsi's Islamist supporters.
It means hundreds of thousands of Brotherhood members now face prison sentences if they hold demonstrations or are found in possession of the movement's recordings or literature.
The designation also means Brotherhood leaders currently on trial face possible death sentences if found guilty.
In Washington, the Pentagon voiced concern at the attack.
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Defense Secretary Chuck Hagel "expressed his condolences for the loss of life and injuries from the bombings in Mansoura, Nasr City, and Sharkiya province," a spokesman said of Hagel's phone conversation with Egyptian Minister of Defense General Al-Sisi.
Hagel "condemned the attacks and offered the assistance of the Department of Defense to help the Egyptian government investigate," the US statement said.
"Hagel also expressed concerns about the political climate in advance of the constitutional referendum, including the continued enforcement of a restrictive demonstrations law."
On Sunday, police in the Mediterranean city of Alexandria arrested three people, including two minors, after they searched a printer's and found propaganda material backing the Brotherhood and opposing the security forces.
The crackdown on Morsi's supporters since the army ousted him on July 3 has seen more than 1,000 people killed and thousands of his backers arrested.
But the Brotherhood has still held demonstrations across Egypt, especially over the past three days, with at least seven people killed in clashes between Morsi's supporters and opponents.
Universities also saw severe unrest this week, and on Saturday a 19-year-old student was shot dead at Al-Azhar's Cairo campus, where pro-Morsi students have staged regular protests.
The students entered the commerce faculty of the university during an exam and set it alight, before police burst into the campus and fired tear gas.
A police official said 101 students were arrested and the fire on the first two floors of the building was brought under control.
The violence came a day after five people were killed in clashes across Egypt, as police stamped down hard on Brotherhood demonstrations.
On Sunday, student Brotherhood backers torched an exam hall at Al-Azhar University, security officials said, adding that 27 "rioters" were arrested.
The interim authorities have decapitated the 85-year-old Brotherhood, imprisoning Morsi and most of the movement's leadership and putting them on trial for allegedly colluding with militants.
More than 100 soldiers and policemen have been killed in the Sinai since Morsi's ouster.
Egypt's first democratically elected president, Morsi ruled for just one turbulent year before the military overthrew him following mass protests demanding his resignation.