Pirates near Somalia’s coastline attacked a cargo ship Wednesday 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. If so, it would be the sixth hijacking over the past week.

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 approximately 7:30 a.m. local time. The nearest U.S. Navy warship was about 300 nautical miles away at the time, 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 was owned and operated by a Maersk subsidiary in Norfolk, Virginia, Maersk spokesman Michael Storgaard said. He would not provide any details about the security arrangements on board the Maersk Alabama.

“We have very strict policies on the vessel … crews are trained to handle these types of situations,” Storgaard said from Maersk’s headquarters in Copenhagen, Denmark.

He said the company is in the process of contacting the crew members’ relatives and setting up assistance for them.

“That is at this moment our primary concern,” Storgaard said.