LONDON—The cost of kidnap and ransom insurance today is 10 times as expensive as it was in October 2008 for ships transiting the piracy hotbed of the Gulf of Aden, according to Aon Risk Services.

More shipowners are inquiring about kidnap and ransom cover, for which a shipper now could pay up to $30,000 in premium for $3 million in coverage for one trip through the Gulf of Aden, the London unit of Chicago-based Aon Corp. said in a statement.

The Gulf of Aden, off the coast of Somalia, is the site of a dramatic uptick in pirate attacks over the last couple of years. Ships in the Indian Ocean must traverse the Gulf of Aden to reach the Suez Canal or steam around the Cape of Good Hope, a longer and much costlier trip around the southern tip of Africa.

In 2008, pirates hijacked 49 ships and took 889 crew members hostage, according to the International Maritime Bureau's Piracy Reporting Center.

Attacks by Somalian pirates dropped considerably in January and February, but they picked up markedly in March. After a total of 14 attacks and one successful hijacking in the first two months of the year, there were 25 attacks with four successful hijackings in March alone, according to the piracy center. So far in April, shippers have suffered at least six attacks with four successful hijackings near Somalia, according to the center.

Security experts say the return of frequent attacks is due in part to the waning monsoon season, which had curtailed pirate activity earlier in the year. Security experts also say the presence of about 15 naval war ships devoted at least in part to combating piracy in the Gulf of Aden has pushed the pirates to operate farther south in the Indian Ocean, where the U.S.-flagged Maersk Alabama was attacked this week.

"I think the message here is that there aren't any really clear limits on the range and ambition of these groups," said David Hunt, head of research and development at Exclusive Analysis Ltd., a London-based intelligence firm that forecasts political risks. "Clearly any ships routing from the Gulf (of Aden) down to the Cape (of Good Hope) will probably want to be keeping the maximum distance they can from the Somali coast."

Somali pirates have typically returned the ships, cargo and crew unharmed in exchange for ransoms between $1 million to $3 million, according to published reports. Traditionally, ransom has been paid by the hull insurance policy, although marine underwriters say the London market has begun to shift piracy coverage to war risk insurance. Kidnap and ransom policies offer additional cover that can include consultant and negotiator costs and medical care, in addition to ransom payment.

About 70% of shipowners are selecting kidnap and ransom policies specifically for the Gulf of Aden as well as two other piracy hot spots—the Gulf of Guinea near Nigeria and the Malaccan Straits near Indonesia—Aon said in the statement.