(CNN) — Pirates off Somalia’s coast on Wednesday attacked a cargo ship with a crew of at least 20 U.S. nationals, according to the company that owns the vessel.

It is believed that the U.S.-flagged Maersk Alabama was subsequently hijacked, according to a statement from Maersk Line Ltd.

The vessel was en route to Mombasa, Kenya, when it was attacked about 500 kilometers (310 miles) off Somalia’s coast, the statement said.

U.S. government sources said the incident happened at about 7:30 a.m. local time. The nearest U.S. Navy warship was about 300 nautical miles away at the time of the hijacking, they said. On Tuesday, the U.S. Navy issued another notice warning mariners that the Somali piracy activity was extending hundreds of miles offshore.

The cargo ship is Danish-owned. No action has been taken so far, a spokesman for the U.S. military’s 5th Fleet in Bahrain said.

“There is a task force present in the region to deter any type of piracy, but the challenge remains that the area is so big and it is hard to monitor all the time,” 5th Fleet spokesman Lt. Nathan Christensen said.

The attacks, which took place south of the area patrolled by U.S. and coalition ships, shows pirates are changing their tactics and taking advantage of tens of thousands of square miles of open water where fewer military ships patrol, according to U.S. military officials.

“They [pirates] are going where we are not, they are looking for targets where there is limited coalition presence,” according to a U.S. military briefing document shown to CNN.

Coalition ships mainly patrol in the busy sea lanes of the Gulf of Aden between Yemen and northern Somalia as ships come out of and head toward the mouth of the Red Sea.

“Despite increased naval presence in the region, ships and aircraft are unlikely to be close enough to provide support to vessels under attack. The scope and magnitude of the problem cannot be understated,” according to a news release from the U.S. Navy.

Between January and February, only two pirate attacks were reported off the east coast of Somalia, according to the International Maritime Bureau, which tracks piracy attacks worldwide.

In March, attacks in the same area spiked to 15, according to the bureau, and the attacks have continued into April.

On Monday, pirates seized a British-owned cargo ship in the Gulf of Aden. Also on Monday, a fishing trawler was hijacked and used to hijack other fishing vessels in the area, the bureau said.

Pirates typically use small boats with a limited range to attack ships just a few miles off the coastline.

The new warning says recent attacks have occurred hundreds of miles off the coast, suggesting that pirates are using more “mother ships” — a practice of using bigger boats with longer range to launch smaller pirate ships from farther out to sea, according to Pentagon officials.

The U.S. Department of Transportation’s Maritime Liaison Office also issued a warning to mariners April 1.

“Recent activity suggests that pirate activity off the east coast of Somalia has increased. Attacks have occurred more than 400 nautical miles offshore,” according to the warning.

The warning suggests ships traveling along the coast of Somalia and Kenya move to the east side of the Seychelles Islands and Madagascar, hundreds of miles east of those coastlines.

Pentagon officials say pirates are holding 15 ships off the Somali coast. And according to U.S. Navy statistics, pirates attacked four ships between Saturday and Monday.

The area involved — off the coast of Somalia and Kenya as well as the Gulf of Aden — equals more than 1.1 million square miles, roughly four times the size of Texas, or the size of the Mediterranean and Red Sea combined. The length of the Somali coastline is roughly the same length as the entire Eastern Seaboard of the United States, according to U.S. Navy statistics.

“We continue to highlight the importance of preparation by the merchant mariners and the maritime industry in this message,” Vice Adm. Bill Gortney, commander of the Combined Maritime Forces, said in a statement.

“International naval forces alone will not be able to solve the problem of piracy at sea. Piracy is a problem that starts ashore,” he said.

This year, the U.S. Navy started Combined Task Force 151, a multinational coalition that uses naval ships to fight piracy in the Gulf of Aden region. Navy officials say about 12 to 15 coalition ships are patrolling in the Gulf of Aden and the off the coast of Somalia.