RICHMOND, Va. -- A judge on Tuesday dismissed piracy charges against six Somali men accused of attacking a Navy ship off the coast of Africa, concluding the U.S. government failed to make the case their alleged actions amounted to piracy.

The dismissal of the piracy count by U.S. District Judge Raymond A. Jackson tosses the most serious charge against the men, but leaves intact seven other charges related to the alleged April 10 attack on the USS Ashland in the Gulf of Aden. A piracy conviction carries a mandatory life term.

Defense attorneys argued last month that the Ashland defendants did not meet the U.S. legal definition of piracy because they did not take command of and rob the amphibious dock landing ship.

Jackson agreed in his ruling, finding that the government "failed to establish that any unauthorized acts of violence or aggression committed on the high seas constitutes piracy as defined by the law of nations."

Jackson, who issued the ruling from Norfolk, wrote that the government was attempting to use "an enormously broad standard under a novel construction of the statute" that would contradict a nearly 200-year-old Supreme Court decision, United States v. Smith.

The six are accused of attacking the Ashland in a skiff, though they claim they were ferrying refugees. The Ashland is 610 feet long and designed to carry hovercraft and other vehicles for amphibious assaults. The skiff was destroyed by one of the ship's 25mm cannons. One occupant of the skiff was killed and several others were injured.

Attorneys for five other Somali defendants accused in a similar attack on the USS Nicholas are also seeking dismissal of the piracy count, citing similar arguments. A hearing is scheduled for Sept. 9 before a different judge in Norfolk.

Government prosecutors did not immediately respond to an Associated Press request for comment.

"The bottom line is there's no piracy because there was no robbery at sea," said Robert Rigney, who is representing Mohammed Abdi Jamah.

The government cited vague international in its attempt to shore up the argument, he said.

"Piracy under the law of nations doesn't give you a real interpretation of what piracy is," he said. Rigney expects the government to appeal to the 4th U.S. Circuit Court of Appeals in Richmond.

At a hearing late last month, Jackson signaled that he was skeptical of the government's piracy accusation, repeatedly questioning prosecutors on what actions constituted piracy.

In his ruling, Jackson said the U.S. definition of piracy has remained consistent through the years, while international definitions are still subject to dispute among experts.

"Given the flexible manner in which international sources treat the definition of piracy, and that these sources inherently conflict with Supreme Court precedent, the court's reliance on these international sources as authoritative would not meet constitutional muster and must therefore be rejected," he wrote.

"The Smith definition of piracy as sea robbery, on the other hand, is clear and authoritative," he wrote.

The five Somali men being prosecuted separately are accused in an alleged assault on the frigate USS Nicholas on April 1, west of the Seychelles. Both Virginia-based ships were part of an international flotilla patrolling pirate-infested waters.

All 11 Somali defendants have pleaded not guilty and are being held until trials this fall. The remaining charges carry terms of 10, 20 and 30 years.