NAIROBI, Kenya — Somali pirates are becoming increasingly violent in their attacks on foreign vessels experts warned in the aftermath of the killings of four Americans who were hijacked last week.

Jean and Scott Adam, a retired California couple, and their friends Phyllis Macay and Bob Riggle, both from Seattle, died Tuesday when a gunfight erupted while the U.S. military was attempting to negotiate their release.

“This does not normally happen,” said Andrew Mwangura, head of the East African Seafarers’ Assistance Program based in the Kenyan coastal city of Mombasa.

Mwangura told GlobalPost that with more than a dozen armed pirates aboard a small boat shadowed by U.S. warships tensions aboard the yacht would undoubtedly have risen. “Misunderstandings can happen among pirates when there are a big number of them in a small boat facing shortages of food and water,” he said.

Somali pirates are currently holding 33 vessels and 712 hostages according to figures compiled by the International Maritime Bureau (IMB) Piracy Reporting Center. The IMB says that there have been 48 attacks and 11 successful hijackings this year.

International navies patrolling in the Gulf of Aden and Indian Ocean have noted that Somali pirates are behaving more violently in recent months, firing assault rifles and rocket-propelled grenades at targeted vessels and treating hostages with greater aggression.

“There is a really unpleasant spike in the violence and pressure tactics that pirates seem willing to use,” said Roger Middleton, a piracy researcher at London’s Royal Institute for International Affairs.

Middleton told GlobalPost that hostages have been tied up and hung from ceilings and that gunshots have been fired during negotiation phone calls to intimidate relatives and employers.

“The stakes are rising and if the pirates are trying to make a $9 million ransom instead of $1 million ransom they are going to use every tactic available to them,” said Middleton.

The navies have also taken a more combative stance. In separate incidents in January Malaysian and South Korean commandoes launched rescue raids on hijacked ships, rescuing crew and killing pirates.

Experts put the change down to a number of factors. They say the piracy has proved so successful (the average ransom has more than doubled in the last year and is now around $5 million) that criminal gangs and militants are entering the business bringing with them a greater willingness to use violence.

In the past most pirates were fishermen with knowledge of the seas but increasingly they are simply armed men on boats who, thanks to naval patrols, travel further out to sea. When they encounter a vessel a successful hijacking might be the only way home, as well as the only way to win a ransom.

“There is a change in the nature of the individuals doing the attacks, from fishermen to fighters,” said Alan Cole, coordinator of anti-piracy programs for the United Nations Office for Drugs and Crime (UNODC) in Nairobi. “As a result we’re seeing a higher mortality of pirates at sea, and when they do attack they are more desperate,” said Cole who called the development “deeply concerning.”

Soon after the 58-foot yacht, the S/V Quest, was pirated off Oman on Feb. 18, a NATO warship deployed as part of an international anti-piracy force captured the pirate's mother ship, leaving the pirates stranded aboard the yacht.

Four U.S. Navy warships began shadowing the yacht: an aircraft carrier, a guided-missile cruiser and two guided-missile destroyers.

A standoff pitching four warships against a single yacht with hostages illustrates the asymmetrical nature of the fight against piracy that was thrown into sharp relief in April 2009 when a handful of pirates detaining an American ship’s captain aboard a life raft held off the U.S. Navy for days.

That deadlock was broken when Navy SEAL snipers shot and killed three pirates and freed Capt. Richard Philips. A fourth pirate, Abdiwali Abdiqadir Muse, was tried in a New York court, found guilty and sentenced to 33 years in jail just days before the Quest was hijacked.

On the evening of Feb. 21 two pirates were taken aboard the destroyer USS Sterett for negotiations, but the following morning everything unraveled. "The intent always had been that this would be a negotiated process and not ever going to a point where we actually had gunfire,” said Adm. Mark Fox, commander of the U.S. Navy 5th Fleet, based in Bahrain.

But at 8 a.m. the next day a rocket-propelled grenade was fired at the Sterett 600-yards away “with absolutely no warning,” according to Fox. Immediately afterward gunfire erupted inside the Quest.

“U.S. [special operations forces] closed in on the Quest in small boats and boarded the yacht,” said Adm. Fox. “As they responded to the gunfire, reaching and boarding the Quest, the U.S. sailors discovered that all four hostages had been shot by their captors. Despite immediate steps to provide life-saving care, all four of the American hostages died of their wounds.”

The U.S. commandoes killed two pirates in hand-to-hand combat once aboard the yacht — one with a knife, the other with a gun — found two pirates already dead suggesting that a dispute may have broken out, and arrested 13 others in addition to the two already aboard the American destroyer.

All 15 surviving pirates are being held aboard the aircraft carrier USS Enterprise while the FBI carries out investigations. The Somalis are likely to be transported to the U.S. for trial where they may face the death penalty.

In a twist that will worry the ship owners who frequently pay out multi-million dollar ransoms to pirate gangs it was reported this week that a deal has been struck between pirates and Al Shabaab, the Somali Islamist militia linked to Al Qaeda.

According to reports pirate gang leaders in the coastal town of Haradhere have agreed to pay the Shabaab insurgents who control the area 20 percent of future ransom payments, an amount that could run into many millions of dollars and which would be directly funding Al Shabaab's terrorism.