WASHINGTON, April 18 (Reuters) – Commercial ships traversing the Gulf of Aden and the Indian Ocean should be armed to defend themselves against marauding Somali pirates because international warships can't do the whole job and won't be there forever, a top US Navy admiral said.

Seaborne gangs of pirates have stepped up hijack attacks on vessels in recent months, making tens of millions of dollars in ransoms by seizing ships, including tankers, despite the presence of dozens of foreign naval vessels.

''We could put a World War Two fleet of ships out there and we still wouldn't be able to cover the whole ocean,'' said Admiral Mark Fitzgerald, commander of US Naval Forces, Europe and Africa, citing attacks from the Gulf of Aden and the Mozambique Channel to off the coast of India.

Overwhelmed by the scope of the maritime problem, the United States has called for a greater international-led focus on going after the pirate money trail.

Underscoring the financial impact of piracy, Fitzgerald said he was told by Kenyan officials that prime real estate in Mombasa and Nairobi were being ''bought up by rich Somalis'' who lead clans which control piracy syndicates. He cited a similar investment trend in Ethiopian property.

''The US can't go this alone,'' he said.

Speaking to reporters at the Pentagon, Fitzgerald said it was ''incumbent upon the vessels who are sailing the high seas to either protect themselves or accept the dangers.''

Asked if he would recommend that commercial ships arm themselves, Fitzgerald said: ''I think they should.''

Some ships already have armed guards on board. Others are using protective devices to try to keep pirates at bay.

''Commercial ships should take appropriate protections ... because we cannot offer 100 percent guarantees of protection as the ships go through,'' Fitzgerald said, putting the onus on the maritime industry to decide ''how seriously they want to take this on.''