An international naval coalition is stepping up its presence in the Bab Al Mandeb strait in response to attacks on vessels passing Yemen’s rebel-held Red Sea coast and an increase in pirate activity.

The strait, which connects the Red Sea to the Gulf of Aden, is a key passage for world trade.

"Recent attacks against merchant shipping in the Gulf of Aden and Bab Al Mandeb have highlighted that there are still risks associated with transits through these waters," the Combined Maritime Forces said on Monday.

"In response to these threats, the Combined Maritime Forces will be increasing the naval presence" in the western part of the Gulf of Aden, it said.

The 31-state naval alliance, based in Bahrain and led by the United States, oversees security in some international waters including the Gulf of Aden.

A small boat exploded late last month "for an unknown reason" in a thwarted attack on a tanker in the area, the alliance said.

Earlier this month, an oil tanker came under fire while passing through the Bab Al Mandeb strait into the Red Sea, according to the Saudi-led coalition supporting Yemen’s government in its fight against the Iran-backed Houthi rebels.

The coalition said three rocket-propelled grenades were fired at the Marshall Islands-flagged tanker from a boat. None of the crew was injured and the tanker sailed on into the Red Sea, it said.

In January, the rebels attacked a Saudi frigate off the Yemeni coast, killing two sailors in what the coalition said was a suicide attack.

In September and October, two US warships and a UAE vessel contracted to the coalition were targeted by missile fire from rebel-held territory.

The head of US Central Command, General Joe Votel, warned in March that coastal defence missiles, radar systems, mines and explosives-laden boats deployed by the rebels posed a threat to shipping in the strait.

The Gulf of Aden has also seen a number of pirate attacks this year, including the attempted hijacking on April 8 of a merchant ship heading to Aden. The attempt was foiled in a joint operation by Indian and Chinese naval vessels in the area.

A few days earlier, pirates seized an Indian dhow that was en route from Dubai to Bosaso in Somalia’s Puntland region.

And in March, an oil tanker was seized off the coast of Somalia in the first successful hijacking of a commercial ship by Somali pirates since 2012. 

At their peak in 2011, pirates launched 237 attacks off the coast of Somalia and took hundreds of hostages, according to the International Maritime Bureau.

The number of attacks decreased sharply after a joint international effort to patrol shipping routes through the Gulf of Aden and off the east coast of Africa. AFP