CAIRO, Egypt – The U.S. Navy bolstered its force of warships off Somalia on Monday, intensifying its watch over Somali pirates holding a hijacked Ukrainian-operated vessel with crew members, arms and tanks aboard.

Lt. Nathan Christensen, a spokesman for the U.S. Navy 5th Fleet, said “there are now several U.S. ships” within eyesight of the hijacked ship, Faina, which according to the Kenyan government was bound for Kenya when it was seized last week. The pirates are negotiating for ransom with the vessel’s owner.

Speaking by telephone from Bahrain, Christensen declined to say how many other U.S. warships had joined the USS Howard, a guided-missile destroyer, off Somalia. The U.S. ships were staying in international waters off Somalia, Christensen said, while the Somali pirates kept the Faina within the 12-mile territorial bounds of Somali waters.

U.S. sailors remained close enough to see the ship and had established bridge-to-bridge contact via radio, he said.

Somali pirates hijacked the Faina on Thursday, seizing its 21 Ukrainian, Russian and Lithuanian crew members and an arms cargo that included 33 T-72 tanks. Kenya said the tanks and weapons were for its military.