Sailors in the round-the-world Volvo Ocean Race have been warned about the increasing danger of pirates off the coast of Somalia and will be monitored by warships patrolling the Indian Ocean, organisers have said.

For the first time in the 35-year history of the race, the course will take the yachts to China and India – and efforts have been made to keep them well away from the Gulf of Aden, which has seen dozens of pirate attacks this year.

“All 88 sailors were briefed on the risks of piracy,” race communications director Marcus Hutchinson said.

Organisers have drawn a diagonal line through Mauritius and anything northwest of the line is considered off-limits. “But even if they cross this line, they will still be 650 miles from Somalia,” Mr Hutchinson said.

The battle with pirates operating off the coast of Somalia grew yesterday when raiders seized two more ships but lost one of their own in a firefight with the Indian Navy. The International Maritime Bureau described the situation as “out of control”.

Saudi Arabia confirmed that pirates have demanded a £25 million ransom for the freeing of the Sirius Star supertanker, seized at the weekend with her crew of 25 and a cargo of oil worth $100 million (£65 million). A ten-day deadline was set by the pirates.

Two more vessels, a Thai fishing boat with a crew of 16, and a bulk carrier, believed to be Greek, with an unknown number of people aboard, were seized by pirates in the Gulf of Aden yesterday, bringing the total to nine vessels in 12 days.