Gunmen have shot dead an army officer in the Nile Delta, Aswat Masriya reported.

Unknown assailants opened fire on Lieutenant Sherif Ahmed late on Sunday in the Mediterranean town of Damietta, a security source said. He was riding in his car along with his fiancée in the northern part of the city.

Investigations are under way as it was not immediately clear if the killing was criminal or political.

Since the ouster of Islamist president Mohamed Morsi last summer, a militant insurgency by Sinai-based fighters has gathered pace, killing hundreds – mostly policemen and troops in bombings and shootings.

The violence has more recently spilled over into the capital Cairo and the Nile Delta.

Over the past week, Cairo has been hit by a string of small explosions and militant attacks have killed at least ten nationwide and injured several others almost a month since Abdel-Fattah El-Sisi, the former army chief who led Morsi's ouster amid massive protests, was sworn-in as president. 

Ten days ago, four police personnel were killed by militant Islamists in the town of Rafah in North Sinai, where the army is waging a broad campaign to root out militant insurgents.