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

In the attack on the German ship, pirates apparently mistook the FGS Spessart vessel for a commercial ship when they opened fire on it on Sunday afternoon, a US Navy spokesman said.

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The German sailors returned fire and pursued the skiff while also calling in for support. Several naval ships — including a Greek and a Dutch frigate, a Spanish warship and the USS Boxer — sped to the area while a Spanish marine aircraft and two US 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. They were later released.

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Germany's Ministry spokesman Christian Dienst said no one was injured in the attack, the first on a German naval ship in this area.

Christensen said that while the casualty-free operation "showcased the incredible international naval capabilities" it also "highlighted the complexity of counter-piracy operations." 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 1.1 million square miles, 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 defense against pirates.