LONDON (Reuters) - Somali pirates are expected to step up attacks on merchant vessels as better weather allows them to operate more easily at sea, the U.S. government warned.

Seaborne gangs have already increased their attacks in recent months, making tens of millions of dollars in ransoms from seizing ships, including tankers and dry bulkers, in the Indian Ocean and Gulf of Aden. Assailants hijacked a number of vessels this week alone.

"Vessel operators should anticipate an increase in piracy attacks from March through May ... when calmer weather favourable for small boat activity will prevail," The U.S. Department of Transportation's Maritime Administration said.

Foreign navies have been deployed off the Gulf of Aden since the start of 2009 and have operated convoys, as well as setting up a transit corridor across dangerous waters.

But their forces have been stretched over the vast expanses of water, leaving ships vulnerable.

"Despite the increase in presence and effectiveness of naval forces in the region, as well as the effectiveness of defensive and protective measures, pirate activity has continued," the Maritime Administration said in an advisory on Monday.

Senior U.S. admiral Mark Fitzgerald has told Reuters Somali pirates have extended their range, snatching ships as far as the Mozambique Channel and off the coast of India.

India's Directorate General of Shipping said that small Indian trade boats, known as dhows, had become the "preferred prey" of pirates in recent months.

To ensure the security of dhows, it has ordered that all Indian registered vessels be prohibited with immediate effect from operating in waters south or west of the line joining Oman's port of Salalah and the Maldives capital Male.

It remained unclear how easy it would be to pass the latest advisory on to dhow operators.

The shipping directorate said in a statement issued on Wednesday that while dhows did not elicit ransoms as high as other merchant vessels, they were easy to seize and could be used as mother ship for pirates operating further off the coast.

It said many of the maritime agencies also lacked information on dhow traffic along the east coast of Africa.

"Dhows are more difficult to track than other ships because they do not have the requisite technical equipment and normally the piracy monitoring agencies do not even know that the dhow is hijacked."

India said this week it was trying to trace the whereabouts of nearly 100 sailors on seven Indian vessels taken captive by Somali pirates.