Security forces aboard a U.S. naval vessel fired warning shots toward two approaching small boats off the Somali coast Tuesday, the U.S. military said Wednesday.
The rounds landed in the water, prompting the boats to turn around, and no casualties were reported, the military news release said.
It is unclear whether the boats were trying to attack the 41,000-ton USNS John Lenthall, the military said.
"It is clear they were not following the international rules of the road observed by mariners around the globe," it said.
The release noted that the location of the incident, the types of boats involved and the maneuvering were all "consistent with reports from previous attacks on merchant vessels in the region."
The USNS John Lenthall is one of 14 "fleet replenishment oilers" in the Military Sealift Fleet Support Command, according to a U.S. Navy Web site. Oilers refuel Navy ships at sea and any aircraft they may be carrying.
Attacks by pirates have increased dramatically off the northern coast of Somalia in the past year, prompting the United States and other nations to step up patrols in the region.
In May, the U.S. Navy warned merchant ships to stay at least 200 miles off the Somali coast. But the U.S. Maritime Administration warns that pirates sometimes issue false distress calls to lure ships closer to shore.
The pirates are often armed with automatic rifles and shoulder-fired rockets, according to warnings from the agency.