On a rocky volcanic outcrop set in the deep and treacherous waters of a vital strait linking Europe to Asia, Yemen’s coastguard is building a base to help secure one of the world’s busiest waterways.

Somali pirates trawl the sea south of the Bab Al Mandab strait off Yemen’s coast, and in recent months have stepped up attacks on tankers, cargo ships and fishing vessels in defiance of a major crackdown by navies from at least a dozen countries.

But Yemen has deeper worries about security off its coast after a resurgent Al Qaeda arm called for a blockade of the strait between the Red Sea and the Gulf of Aden, through which 25,000 ships - 7% of world shipping - pass each year. “The geographical nature of Bab Al Mandab, of the coast and the land, its beaches and islands, makes it very sensitive security-wise,” Mohamed Mubarak bin Aefan, head of Aden port management, said.

Militants would struggle to block the strategic strait, experts say, but the shipping industry is still worried about possible attacks off Yemen’s southwest coast. A Yemeni official said France was helping it build the base with the hope it would have a dual use in combating both piracy and Al Qaeda.

Yemen has seen its ports and waters targeted before.

The US government warned ships sailing off Yemen’s coast in March of a risk of Al Qaeda attacks similar to a suicide bombing of the US warship Cole in 2000 that killed 17 US sailors in Aden’s port. Two years later, Al Qaeda hit a French supertanker in the Gulf of Aden, south of Bab Al Mandab.

Worries over the strait, through which around 3mn barrels of oil bound for Europe and the US are shipped daily, were further stirred when Yemen boosted security on its coast against possible militant attacks.

Yemen became a top Western security concern after a resurgent Yemeni Al Qaeda arm claimed a failed bomb attack on a US-bound plane in December, so alarming Washington that it has cranked up security assistance to the impoverished country.

In another bold attack in June that Yemen blamed on Al Qaeda, gunmen killed 11 people at the southern regional headquarters of a Yemeni intelligence agency in Aden, the deadliest attack in Yemen since the Cole bombing.

But the group’s call earlier this year for a blockade of Bab Al Mandab to cut off US shipments to Israel does not mean Al Qaeda is capable of such an operation, said Jim Cameron, senior analyst at Stirling Assynt.

“It’s certainly a real threat although I think it’s probably more an aspiration rather than a capability at the moment.”

In addition, it would not be easy to completely close off the 22km strait, experts say.

“The strait is wide and the currents are strong and complex, so it would be difficult to actually block it in a physical sense,” said Roy Facey, port adviser to the Port of Aden.

“A Yemen coastguard base to support maritime interventions, and the ability of Yemeni forces to control any high land overlooking the strait gives me a lot of confidence that the threats we hear of would be very difficult to implement.”

But calls to close Bab Al Mandab still impact sentiment in the region’s shipping industry, reeling from pirate attacks, said Hisham al-Saqaf, general manager of shipping and marine services firm Gulf Agency Company (GAC) Yemen.

“I don’t know how they would do it but of course this is a threat and ship owners take these things seriously,” he said.

While Yemen’s Western allies and neighbouring oil exporter Saudi Arabia fear Al Qaeda is exploiting instability on several fronts in Yemen for attacks in the region and beyond, piracy is the most burning concern for the shipping industry.

Somali pirates are making millions of dollars in ransoms by seizing ships, including tankers and dry bulkers, in the Gulf of Aden and the Indian Ocean. More than 15 ships and hundreds of sailors are being held off Somalia.

Business in the region’s ports has been badly hit and shipping lines are having to fork out millions of dollars in higher insurance rates, extra security costs and elevated crews’ wages, all this despite the strong international naval presence.

The US estimates that every day some 30 to 40 warships are involved in counter-piracy efforts from the EU, Nato and the US as well as China, Russia, India, Malaysia, South Korea and Japan.

Serious security threats to ships at the Bab Al Mandab waterway would have global implications for the industry and could prompt an even stronger international military intervention, industry experts say.

The GAC’s Saqaf, whose tanker business is 50% down compared to 2008, said the naval intervention had improved security, but more needed to be done.

“At the end of the day I want a peaceful passage, a peaceful waterway for ships to sail and to come to our ports. We need the business,” he said.