CAIRO, (Reuters) - Unknown assailants attacked a natural gas pipeline in the Sinai on Tuesday, Egyptian security sources told Reuters, raising concerns of instability as the country pushes through with a roadmap for political transition to democracy.

The blast took place in the central region of Sinai on a pipeline that carried natural gas to an industrial area.

There were so far no reports of casualties and security forces are scanning the area to investigate the cause of the blast, the sources said.

Egypt has been struggling to maintain stability in the country of 85 million people since the army ousted Islamist President Mohamed Mursi, the country's first elected leader, on July 3 following mass protests against his rule.

Supporters of Mursi have called the army move a military coup and staged regular protests demanding his reinstatement. Militant attacks in the Sinai region have intensified since then.

Hundreds were killed in Cairo when security forces stormed two pro-Mursi camps in August and thousands more were arrested, including Mursi, as the government launched a crackdown against his Muslim Brotherhood group, which it accused of violence.

The Muslim Brotherhood denies any link to violence, but last week the government declared them a terrorist group, accusing them of carrying out a suicide attack that killed 16 people in the Nile Delta.

Sinai-based Islamist militant group, Ansar Bayt al-Maqdis, had claimed responsibility for the Nile Delta attack, as well as several previous attacks, including a failed attempt to assassinate the interior minister with a car bomb in Cairo in September.

Egypt plans to hold a constitution referendum in mid-January, a major milestone in its roadmap to a transition to democracy that could see presidential and parliamentary elections next year.

On Tuesday, security forces arrested the son of a prominent Brotherhood leader, Anas Beltagi, and the former spokesman for the presidency under Mursi, Yasser Ali. They were arrested over accusations of inciting violence, judicial and interior ministry source said. (Reporting by Asma Alsharif. Additional reporting by Youssry Ahmad. Editing by Andre Grenon)