Story highlights The acting PM calls Muslim Brotherhood a terrorist group but stops short of blaming it

Two blasts hit an Interior Ministry building in Mansoura

Blasts come as nation prepares for a referendum on a new constitution

More than 130 people were injured, media reports

At least 12 people were killed and more than 130 injured Tuesday when two explosions hit an Interior Ministry building in the Nile Delta city of Mansoura, Egyptian media reported.

Previously, the semi-official Al Ahram news agency reported 14 dead, citing the country's Health Ministry. It was not immediately clear why the death toll was lowered.

Among the people injured is the head of the security directorate of Dakahlia governorate, state-news agency MENA said, citing Health Ministry officials. Part of the building collapsed after the explosion, MENA said.

One blast occurred on one of the top floors of the building and was followed by a car bomb, MENA reported.

Egypt's interim Prime Minister Hazem al Beblawi called it a terrorist incident and said the perpetrators will not go unpunished.

Without directly blaming the Muslim Brotherhood, the party that backs ousted President Mohamed Morsy, he called that party a terrorist group in his statement to the nation.

The Muslim Brotherhood tweeted Tuesday that it "strongly condemn(ed) the cowardly bombing in #Mansoura & express deep condolences to families of the victims #Egypt."

An Egyptian court in September ordered a ban on activities by the Muslim Brotherhood and froze its finances, in a move that diminished its political power.

Witness: Heavy damage at the site

Ahmed El Shabrawy was at his home when he heard a loud blast. He told CNN it took him five minutes to run to the building where he saw a man pulled by other residents from the rubble of the building. He estimated that dozens of cars were damaged, and there was a large area around one of the blast sites were there was "heavy material damage."

At least 10 ambulances had responded to the scene, he said. So many people rushed to a blood donation center that workers had to turn people away, he added.

CNN first learned of the blasts from Twitter.

Upcoming vote on proposed constitution

The blasts come in the lead-up to Egypt's referendum on a new constitution, which will be held January 14 and 15.

The draft constitution would ban religious parties and put more power in the hands of the military.

A military coup in July unseated Morsy, who was democratically elected.

Since then there have been almost daily protests, some of which have ended in violence and other bombings, such as one on December 12 when 20 police recruits were injured in a car bomb attack.

On Sunday, Ansar Jerusalem, a self-proclaimed jihadist group responsible for several terrorist attacks, mainly in the Sinai Peninsula, issued an online statement calling on army and police members to quit.

"With your staying in these institutions from evening to morning, you are incurring the anger of Allah," the statement said.

No group immediately claimed responsibility for Tuesday's bombing.

Morsy, an Islamist leader, has endured a series of legal troubles.

He and 132 others will face trial for escaping from prison in 2011, state media reported Saturday.

Morsy will be tried along with members of the Muslim Brotherhood and Palestinian Hamas, al Ahram reported.

He is also charged with raiding other prisons, and killing soldiers and officers in Rafah, it said.

He has been in custody since his ouster.