The pirates who seized the ship are demanding a $20 million ransom.
The U.S. fears the armaments onboard the Ukrainian vessel may end up with al-Qaida-linked Islamic insurgents who have been fighting the shaky U.N.-backed Somali transitional government since late 2006.
"We maintain a vigilant watch over the ship and we will remain on station while negotiations between the pirates and the shipping company are going on," Christensen told The Associated Press.
Pirates seized the Faina's Ukrainian, Russian and Latvian crew off Somalia's lawless coast on Thursday as it headed to Kenya and anchored the vessel off Somalia's coast near the central town of Hobyo. One crew member has died.
Christensen said U.S. destroyers and cruisers have been deployed within 10 miles of the Ukrainian vessel and American helicopters were circling above.
Sugule Ali, a spokesman for the pirates, said Sunday that "planes" were flying overhead. He said he was speaking from the deck of the Faina via satellite phone - and verified his location by handing the phone over to the ship's captain, who also spoke with the AP.
The 5th Fleet said the ship was headed for the Kenyan port of Mombassa, but that "additional reports state the cargo was intended for Sudan."
Christensen did not specify whether the arms were intended for the Khartoum-based government, or southern Sudan, which was granted a degree of autonomy under a 2005 peace deal that also guaranteed the oil-rich region a referendum on full independence in 2011.
The U.N. has imposed an arms embargo on weapons headed to Sudan's Darfur conflict zone. The ban does not cover other weapons sales to the governments in Khartoum or southern Sudan.
The shipment was destined for southern Sudan - not Darfur - and did not violate the embargo, a Western diplomat in Nairobi, Kenya, who spoke on condition of anonymity because he's not authorized to speak to the press, told the AP.
"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."
Christensen said the Navy maintains "standard bridge-to-bridge communication" with Faina's crew via DHF radio, but stressed that "we are not taking part or in any way facilitating negotiations between the pirates and the shipping company."
Kenyan officials on Monday declined to discuss the destination of the weapons. Ukrainian Defense Ministry spokesman Valentyn Mandriyevsky said the ministry was not dealing in weapons trade and didn't know where the cargo was bound.
A spokesman for Ukraine's arms trader, Ukrspetexport, had no immediate comment.
Western intelligence reports a few days ago said the ultimate destination was Sudan and that Kenya was only the transshipment point, said one Western official, who spoke on condition of anonymity because he was discussing classified material. He said the issue became confused after Kenyan leaders had publicly referred to the tanks as their own.
Christensen said an unspecified number of destroyers and cruisers have joined the San Diego-based USS destroyer Howard within a 10-mile radius of the Faina.
"The safety of the ship's crew and cargo is a paramount concern to us," Christensen said, adding additional warships and helicopters were deployed to prevent the weapons from falling "into the wrong hands."
There have been 24 reported attacks in Somalia this year, according to the International Maritime Bureau's piracy reporting center. Last year, U.S. naval helicopters fired on pirate skiffs tied to a hijacked Japanese tanker carrying 30,000 tons of benzene after they feared that pirates might try to use it as a floating bomb in a middle eastern oil port.
Seizing ships has become an important source of income for pirates in Somalia, which is riven between rival clan-based warlords since they overthrew a socialist dictator in 1991.
