The captured ship in open seas off Somalia
The US navy says it has deployed further vessels to surround a cargo ship loaded with 33 tanks and hijacked by pirates off Somalia last week.
A US navy spokesman said destroyers and cruisers surrounded the Ukrainian ship to stop the tanks and other weapons "falling into the wrong hands".
He also cited reports that the cargo was meant for Sudan - and not Kenya, as claimed by the Kenyan government.
Pirates have said they want a ransom of $20m (£11m) to release the ship.
A spokesman for the US Navy's 5th Fleet, Lt Nathan Christensen, said the new vessels had been deployed within 10 miles (16kms) of the hijacked ship, the Faina.
The Faina had already been surrounded by three warships, including the US destroyer USS Howard.
Another of the ships was from Russia but the nationality of the third was not clear.
"We will maintain a vigilant watch over the ship and remain on station while negotiations take place," said Rear Adm Kendall Card, commander of the task force monitoring the ship.
"Our goal is to ensure the safety of the crew, to not allow off-loading of dangerous cargo and to make certain Faina can return to legitimate shipping."
The Ukrainian-operated Faina, which had been sailing to the Kenyan port of Mombasa, has been moored near the town of Hobyo.
One of the ship's 21 crew is reported to have died from an illness on board.
Earlier a pirate speaking from the Faina by satellite phone had said the pirates were not afraid and had enough food to survive a siege.
Kenya has insisted that the shipment of 33 72-T tanks on board were destined for its military.
But other sources have suggested that they were bound for the autonomous government of South Sudan, in possible contravention of a UN arms embargo.
"We are aware that the actual cargo was intended for Sudan, not Kenya," said Lt Christensen.
Somalia has been without a functioning central government for 17 years and has suffered continual civil strife, with rival armed groups fighting for control.
Islamist insurgents, not known to have links to the pirate gangs, are battling government troops, their Ethiopian allies and African Union peacekeepers in the capital, Mogadishu.
The waters off the coast of Somalia are considered some of the world's most dangerous.
Even ships carrying food aid are often targeted, hampering the delivery of humanitarian supplies to the estimated three million Somalis in need of aid.
France, which has troops in nearby Djibouti and also participates in a multi-national naval force patrol in the area, has intervened twice to release French sailors kidnapped by pirates.
Authorities in Somalia's semi-autonomous region of Puntland say they are powerless to confront the pirates, who regularly hold ships for ransom at the port of Eyl.