Maritime authorities are warning of an increased threat to ships off the coast of Somalia after a cargo ship became the latest vessel to be seized in a suspected act of piracy.

The UK Maritime Trade Operations (UKMTO) raised the threat level in the area to "substantial" as it reported "unauthorised persons" seized the vessel and re-directed it to Somalia's territorial waters on Sunday.

At least four vessels have been targeted in suspected piracy incidents in the last week including a fishing vessel and oil tanker.

Until three years ago piracy had almost disappeared in this stretch of the Indian Ocean once notorious for hijackings, but it has since made a comeback.

"Due to the increased threat of possible PAG (Pirate Action Group) activity, vessels are advised to transit with caution," the UKMTO said in a statement, noting that weather is conducive to small boat operations.

The hijacking on Sunday occurred six nautical miles off Garacad, Somalia, the UKMTO said in a statement. No further details were reported.

The UKMTO also issued a warning over a separate hijacking off the coast of Mareeyo, northern Somalia which occurred on 21 April.

Pirates also hijacked an oil tanker, Honour 25, with 17 crew that was sailing near the Somali coast on 22 April, according to multiple security officials speaking to the BBC.

Under the control of the pirates, the vessel, with a crew of 10 Pakistanis, four Indonesians, one Indian, one Sri Lankan and one from Myanmar, anchored close to the Somali shore between the fishing towns of Xaafun and Bander Beyla.

The UKMTO also reported that a Somali-flagged fishing vessel had been seized on Thursday.

At least two armed people also attempted to board a cargo ship from a small boat on Thursday, the UKMTO reported, although they turned away after the cargo's crew fired warning shots.

Between 2005 and 2012, pirates off the Horn of Africa raked in between $339m (£266m) and $413m (£325) by holding crew members hostage and demanding ransom payments, the World Bank has estimated.