Seven pirates opened fire on a German naval supply ship in the Gulf of Aden but were chased down and captured by an international anti-piracy task force, the U.S. Navy and European officials said Monday.

In the attack, pirates apparently mistook the FGS Spessart supply vessel for a commercial ship when they opened fire on it on Sunday afternoon, U.S. Navy 5th Fleet spokesman Lt. Nate Christensen said.

The German sailors returned fire and pursued the skiff while also calling for support. Several naval ships — including a Greek and a Dutch frigate, a Spanish warship and the USS Boxer — rushed to the area while a Spanish marine aircraft and two U.S. Marine Cobra helicopters joined the pursuit.

Five hours later, Greek sailors reached the pirate skiff, boarded it and seized the seven suspects and their weapons, including assault rifles and rocket-propelled grenades, the Greek navy said.

The suspects were disarmed and transferred for questioning to the German frigate Rheinland-Pfalz where they remained Monday, pending a decision on whether they will be legally prosecuted, Christensen said.

Germany defence ministry spokesman Christian Dienst said no one was injured in the attack, the first on a German naval ship in the area.

Christensen said that while the casualty-free operation "showcased the incredible international naval capabilities" it also "highlighted the complexity of counter-piracy operations."

Anti-piracy forces monitor massive area at sea

Apart from the Gulf of Aden, where the international anti-piracy efforts have been increasingly successful, pirates have also stepped up attacks further south off the eastern Somali coast.

The two areas combined equal more than 2.8 million square kilometres, or roughly four times the size of Texas, said Christensen. "We can't be everywhere at once," he said, adding that merchant mariners must often serve as the first line of defence against pirates.

Meanwhile, Yemen reported that pirates killed a Yemeni fisherman and wounded two others in an attack on a fishing boat Saturday, also in the Gulf of Aden.

The expanse between Somalia and Yemen is one of the world's busiest waterways and the thousands of ships passing through each year have been plagued by pirate attacks.

Somalia has not had a functioning government since clan-based militias overthrew a socialist dictator in 1991 and then turned on each other.

Pirate attacks off its coastline hit unprecedented levels in 2008, when pirates made 111 attacks and seized 42 vessels, mostly in the Gulf of Aden.

Piracy and seizures by naval forces on the rise

Seven ships have been seized so far this year, although there were roughly 10 times as many attacks in January and February 2009 as there were over the same period last year. There have been almost daily attacks in March.

Somali pirates currently hold 11 ships hostage, with the oldest pirate hijacking dating back to last August, Christensen said.

Yemeni fishermen have also been targeted by pirates, and Yemeni media reported about 50 fishing boats were attacked over the past year.

During Saturday's attack on a Yemeni boat, the fishermen were in Somali territorial waters when they came under fire, he said. The fishing boat managed to flee and the body of the dead fisherman was later brought back to the Yemeni port of Mukalla, 560 kilometres southeast of the capital, San'a.

The latest attacks came as the gas tanker Longchamp, hijacked in January, was released on Saturday with 12 Filipino crew and an Indonesian second engineer on board. The company that manages the vessel said they were unharmed.

Also Saturday, the captain of the Norwegian tanker Bow Asir, hijacked off Somalia on Thursday by pirates carrying machine guns, called the owner to say all 27 crew members were safe, according to the Norwegian news agency.