The pirates aboard the Ukrainian-operated Faina are demanding $20m to release the tanks, rifles and ammunition, along with 21 crew members, one of whom has died.
The ship apparently was destined for Sudan when armed pirates overtook it, likely by alighting from a speedboat and clambering up the side of the ship.
"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," Lieutenant Nathan Christensen, a deputy spokesman for the US Navy's Bahrain-based 5th Fleet, told The Associated Press on Monday.
A Western official in Washington, who spoke on condition of anonymity because he was discussing classified material, said the pirates have not been allowed to take anything off the Faina, but they have been "allowed to resupply", apparently referring to items such as food.
The US fears the armaments onboard the Ukrainian vessel may end up with al-Qaeda-linked fighters who have been fighting the shaky, UN-backed Somali transitional government since late 2006, when they were driven out after six months in power.
Christensen said the arms shipment had been destined for Sudan - not Kenya, which had been claiming to be the arms' destination. "We are aware that the actual cargo was intended for Sudan, not Kenya," he said.
The 5th Fleet said the ship was headed for the Kenyan port of Mombasa, but that "additional reports state the cargo was intended for Sudan".
UN officials said there is no blanket arms embargo on Sudan's government but that any movement of military equipment and supplies into the Darfur region would violate a UN arms embargo, if it were not first requested by the government and approved by the Security Council's Sudan sanctions committee.
A Western diplomat in Nairobi, Kenya, who spoke on condition of anonymity because he is not authorised to speak to the media, said the shipment was destined for autonomous southern Sudan - not Darfur - and did not violate the embargo.
Meanwhile, the US Navy said American destroyers and cruisers have been deployed within 16km of the hijacked vessel and that helicopters were buzzing overhead because of "great concern" over the possibility of the cargo falling "into the wrong hands", Christensen said.