More than 200 hostages on ten boats are being held by Somali pirates after a spate of attacks by gangs armed with automatic weapons and rocket-propelled grenades, according to shipping experts.

With shipping companies paying ransoms of up to $1 million (£560,000), the frequency and sophistication of piracy in the Gulf of Aden is increasing, the experts say.

At least 54 vessels have reported attacks so far this year, compared with 44 for the whole of 2007 and 20 in 2006, according to the International Maritime Bureau.

The bureau’s Piracy Reporting Centre says that three large pirate vessels are reported to be operating in the waters off Somalia, leading to six incidents in the past week alone.

The gangs – described by the bureau as increasingly well organised – use mother vessels as a base from which to launch their attacks on merchant ships and yachts. On Sunday, for example, six armed pirates attacked a bulk carrier, which managed to escape. Two days earlier eight pirates in a speed boat chased and attempted to board another carrier, which was saved by an intervention from a navy helicopter dispatched by the US-led coalition patrolling the Gulf of Aden. On Wednesday the coalition had to send a warship to deter yet another armed group.

The bureau’s experts are recommending that owners equip their ships with a 9,000-volt electric fence around the deck so that intruders receive “an unpleasant nonlethal shock”.

The problem is so serious that European foreign ministers agreed this week to set up a unit to coordinate warship patrols off Somalia to protect shipping from pirates, raising the possibility of an EU naval mission to the region. Several EU nations have already sent warships to the area.

Last weekend Le Drennec, a French tuna fishing boat, escaped an attack by pirates using rocket-propelled grenades. After that attack EU foreign ministers approved the plan for a military mission in the Gulf of Aden.

The EU is to set up a unit to provide support and coordination for antipiracy operations by member states.

Michel Barnier, the French Agriculture Minister, said that tuna-fishing boats could ask for protection from the country’s air force.

Most of the ships accosted in the waters off Somalia this year either escaped or were allowed to continue their journey after handing over money, valuables or goods to the pirate gangs.

But 21 have been hijacked and mostly brought to anchor along the coast of the semi-autonomous Puntland region. Of these, 11 have been released, usually after the payment of a ransom. No hostages are thought to have been killed in Somali pirate hijackings.

The ten ships still being held include two flying the Malaysian flag, as well as Japanese, Egyptian, Thai, Iranian, South Korean, Chinese, Nigerian and Panamanian vessels.

The number of hostages in the pirates’ hands is estimated by the bureau at about 230. They come from 15 countries, including the Philippines, Russia, Croatia, India, Pakistan, Italy and Nigeria.

Ransoms vary, but represent a lucrative source of income in impoverished Somalia. Siyad Mohammed, the leader of one of the pirate gangs, said that his group had been paid $750,000 to release a German ship hijacked in May.

The Iranian owners of MV Iran Deyanat, a chemical tanker hijacked in August, paid the $200,000 demanded by the hijackers, but the gang refused to free the ship and crew, saying that the money had been only a first instalment of what was needed.

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