NAIROBI, Kenya (Reuters) -- Somali pirates attacked five ships in the past week in a sharp rise of banditry apparently orchestrated by a mysterious "mothership" prowling the busy Indian Ocean corridor, shipping experts said Friday.Most vessels escaped, but one was commandeered, bringing to seven the total vessels and crews now being held captive by pirates plundering the coastline, the International Maritime Bureau said."Insecurity off the Somali coast has escalated sharply -- it is very worrying," Andrew Mwangura, program coordinator at the Kenyan Seafarers' Association, told Reuters. He said nine ships have been seized.Mwangura said five vessels were attacked in the past week alone including the attempt last Saturday to board the Bahamas-registered Seabourn Spirit, which was carrying 151 Western tourists. (Watch passengers recall their encounter with ocean marauders -- 2:34)Rocket-propelled grenades and assault rifles were fired at the U.S.-owned Spirit by gunmen in two small speedboats, but the ship's captain managed to change course and speed away. (Full story)The northern and southern coastline of Somalia -- Africa's longest -- links trade routes for key commodities like oil, grains and iron ore from the Gulf and Red Sea region down to the Mozambique Channel. Thousands of merchant ships snake down the Somali coast to the Cape of Good Hope every year.Some of the world's leading shipping bodies called on the U.N. International Maritime Organization and the U.N. Security Council to urgently address the issue."We think it most important that this clearly growing threat to the safety of ships on the high seas is taken up at the highest diplomatic level," a joint letter to IMO's Secretary General read."The attacks against shipping off Somalia have direct implications for the security of the world's transport supply chain."Reports of 'mothership'At the center of the wave of recent attacks is a mysterious, so-called mothership that has been spotted three times since late July drifting off the northeast coast of Somalia."We understand that this is the vessel that is launching the speedboats that go to attack the victims," Mwangura said. "We are still trying to discover the name of this ship, its owner, its nationality and the identity of the crew on board."The IMB, which called the situation completely out of control, confirmed a mothership had been involved in the attacks, which took place well off the coast.The piracy watchdog has warned merchant ships to stay at least 200 nautical miles away from the Somali coast -- an admonition that has gone unheeded.After two years of relative calm, 32 pirate attacks have been recorded since mid-March, including raids on ships carrying supplies for the U.N. World Food Program, according to the IMB.Mwangura said among the ships being held hostage by pirates were vessels registered in Thailand, Taiwan, Malta and Ukraine. More than 100 crew members were being held for ransom.Somalia has been ruled by rival warlords since dictator Mohammed Siad Barre was overthrown in 1991. Many of the warlords are believed to run gangs who smuggle drugs, weapons and people by road, sea and air around the region.Piracy is a lucrative and growing offshoot of this trade.On Wednesday, the U.N. Security Council scolded Somalia's squabbling government and urged rival factions to come together to confront the chaos and piracy plaguing the lawless nation.The council expressed "serious concern" about the recent wave of pirate attacks off the coast, and called on regional powers and international bodies to address the problem urgently.