CAIRO, Oct 29 (Reuters) - Egyptian authorities have arrested an Islamist militant jailed over the assassination of Anwar Sadat, for plotting bomb attacks on behalf of al Qaeda since his release from prison in 2012, security sources said on Tuesday.

Nabil al-Maghraby, a former naval intelligence officer who the sources said was a key al Qaeda operative, was arrested on Sunday. He has been linked to an ex-army major who tried to assassinate the interior minister in September, the sources said.

Maghraby had been released from jail after serving 31 years for taking part in the 1981 assassination of Sadat. The Egyptian leader was killed by Islamist members of the military opposed to his 1979 peace treaty with Israel.

Since then, Egypt has been highly sensitive about Islamists infiltrating its armed forces, the biggest in the Arab world.

Maghraby was arrested in the northern Delta town of Qalubiya, 35 km (20 miles) from Cairo.

"He has been arrested for being a part of a terror organization and for planning bomb attacks in the country," one of the sources said.

Authorities believe he was a close associate of former army major Waleed Badr, who blew himself up on September 5 in a failed attempt to kill the interior minister in Cairo.

A Sinai-based Islamist militant group called Ansar Bayt al-Maqdis released a farewell video of Badr urging Muslims to kill government officials in Egypt, a strategic U.S. ally.

Al Qaeda-linked Islamist militants based in the Sinai have stepped up attacks on soldiers and police since the army toppled Islamist President Mohamed Mursi in July.

The majority of the attacks have been carried out in the largely lawless Sinai Peninsula, but some attacks like the one on the interior minister have raised fears that an Islamist insurgency is taking hold elsewhere.

Maghraby was released from jail in 2012 after Mursi pardoned about 100 political prisoners, mostly Islamists. (Reporting and writing By Hadeel Al-Shalchi; Editing by Michael Georgy and Barry Moody)