Spokesman for Egypt's cabinet reportedly blames group for bomb in Mansoura which has killed at least 14 people

Mohamed Morsi's Muslim Brotherhood has been called a terrorist group by a spokesman for Egypt's cabinet, according to a state-run news agency, after the group was blamed for an explosion at a police headquarters 80 miles (130km) north of Cairo that killed at least 14 people early on Tuesday morning.

The Muslim Brotherhood immediately condemned the attack, and said the government was trying to exploit the deaths in order to smear the group.

Over 100 were injured in the blast in Mansoura, Egypt's third city, including the area's police chief, according to the governor of the local province. State television said it was the largest attack in the city's history, and anonymous security officials told state media it had been caused by two car bombs.

Photographs of the building showed significant damaged to at least four of the building's stories.

Egypt's prime minister, Hazem el-Beblawi,distanced himself from the cabinet spokesman's reported comments, declining to blame any group. "Whoever is behind this act is a terrorist and will be brought to justice and punished according to the law," he told a private channel early on Tuesday. "But I don't want to anticipate the incidents."

The attack was the latest in a series of assaults on government facilities and police stations since Morsi's overthrow in July that have left well over 100 police and soldiers dead. The deadliest incident involved the storming of a police truck in August in the northern Sinai peninsular that ended with the execution of 25 police conscripts.

In September, Egypt's interior minister survived an assassination attempt in Cairo that injured at least 19. It was one of several attacks claimed by Ansar Beit al-Maqdis, a jihadist cell based in northern Sinai, where extremists have waged an insurgency against the Egyptian army since July. No tourist sites have been targeted.

Though the Muslim Brotherhood's involvement has never been proven, Egyptian officials and most media outlets have consistently blamed the group for the attacks, claiming that they finance and control the jihadist cells. Legal proceedings based on these allegations were initiated against Morsi himself last week.

The accusations have been used to justify an ongoing crackdown against the group and its supporters, over a thousand of whom have been killed since July, and thousands more arrested. The group was the most powerful political organisation as recently as June – but was pushed out of office in July, and formally banned in September.

On Tuesday morning, the Muslim Brotherhood issued an English-language statement condemning the Mansoura bomb "in the strongest possible terms" and criticised Beblawi for using "the blood of innocent Egyptians through inflammatory statements".

No group has yet claimed responsibility for Tuesday's attack, but a researcher specialising in Egyptian insurgents said it bore the hallmarks of Ansar Beit al-Maqdis, who recently threatened to target those who refuse to leave the security services.

"The tactic and size of the explosion is reminiscent of previous attacks by Ansar Beit al-Maqdis," said David Barnett, a research associate the Foundation for Defence of Democracies, who has been documenting all recent attacks. "Ansar Beit al-Maqdis has used car bombings in a number of its attacks both outside of North Sinai [on 5 September and 19 October] as well as in North Sinai [on 20 November]. Relatedly, in recent months we have seen a greater number of high profile attacks, which suggests an increase in both capability and experience for those responsible."

• This article was amended on 24 December 2013. It originally stated that at least 19 people had been killed.