Eight Egyptian servicemen are missing after Egypt’s navy exchanged fire with unknown gunmen in the Mediterranean sea, in a mysterious and unprecedented skirmish that the military termed a “terrorist incident”.

According to limited details released by the military, the exchange took place at dawn on Wednesday, resulting in injuries to five naval personnel, the capture of 32 assailants, and the destruction of four enemy boats. Pressed for a fuller explanation of events, military sources declined to comment.

The incident happened roughly 45 miles (72km) from the Egyptian coast, according to a state-run news agency, about 30 miles inside international waters – leading to questions about what had brought an Egyptian naval vessel so far from Egypt’s maritime border.

No details about the attackers were released. Jihadi extremists Ansar Beit al-Maqdis (ABM), who this week disclosed their allegiance to Islamic State (Isis), are fighting a land-based insurgency in the northern Sinai peninsular, which borders the Mediterranean. But ABM has never previously been reported to have maritime capacities.

Smugglers use the Mediterranean to traffic migrants and weapons, and the navy is known to occasionally intercept them but the number of boats and individuals reported to have taken part in the incident appeared higher than usually involved in smuggling missions.

At least one of the assailants’ boats was brought back – damaged and burnt – to the Egyptian shoreline, according to Al-Ahram, the main state-run newspaper. Meanwhile, shipping activity continued as normal on Thursday at Egypt’s largest port in Alexandria, a maritime official told Ahram in the same report.

In northern Sinai, ABM’s insurgency continued with five soldiers shot dead by militants on Thursday. A small explosion on Cairo’s metro also caused around a dozen injuries. But the beaches and resorts in southern Sinai remained unaffected.

• Additional research: Manu Abdo