A record four ships were seized in 48 hours last week off the anarchic Horn of Africa nation, meaning Somali pirates are currently holding hostage four cargo vessels, two tankers and a tug boat, along with about 130 crew members.

The spike in attacks at sea has coincided with a rise in assaults on land by radical al-Shabaab insurgents, including the capture on Friday of Somalia's strategic southern port Kismayu.

The United States say al-Shabaab is a terrorist group with close ties to al Qaeda. Experts say some of the businessmen and warlords who command the pirates are also funding the rebels.

"The entire Somali coastline is now under control of the Islamists," Andrew Mwangura, head of the East African Seafarers' Assistance Programme, told Reuters in an interview.

"According to our information, the money they make from piracy and ransoms goes to support al-Shabaab activities onshore."

Piracy has been rife off Somalia since warlords overthrew dictator Mohamed Siad Barre in 1991. Experts say at least 30 ships have been hijacked off the coast so far this year -- and the attacks have hit unprecedented levels this month.

"It's crazy. We have never seen anything like it in our years of tracking them," Mwangura said. "They've broken all records for piracy in this region and indeed the whole world."

RICH REWARDS

The main lure is money. Most of the hijacked ships have brought ransoms of at least $10,000, and sometimes much more.

Many pirates, particularly in the northern Puntland region, have quickly become local celebrities, flaunting their newfound cash by building palatial beachside villas, marrying extra wives or roaring around its dusty towns in flashy cars.

And that has attracted many young men desperate for work in one of the poorest countries on the planet.

"Back in 2005, there were just five Somali pirate gangs, with fewer than 100 gunmen," Mwangura said.

"Now that youths who used to work as bodyguards for warlords or militia for the government see the rewards available at sea, our estimate is that there are between 1,100 and 1,200 pirates."

Thursday -- a day before al-Shabaab fighters seized Kismayu following battles that killed at least 70 people -- was the worst day on record for piracy in Somali waters.

In the space of one day, gunmen hijacked a German cargo ship, an Iranian bulk carrier and a Japanese-operated tanker. That came after a Malaysian tanker laden with palm oil was seized in the same area on Wednesday.

The pirates are also holding a Thai cargo ship, a Nigerian tug boat and a Japanese-managed bulk carrier.

Mwangura said the captors of the Nigerian vessel had demanded a $1 million ransom to free it and its 10 crew.

He said there were also reports some Malaysian and Filipino hostages on board two of the other hijacked vessels might have been badly hurt by gunfire. But he said that was not confirmed.

His organisation advises all shipping using the area to maintain a strict lookout for pirates around the clock, and to be especially wary of any small boats that approach them.