A U.S. warship is tracking the vessel but there has been no decision about intercepting it, U.S. Defense Department officials said.
"I think we're looking at the full range of options here," Pentagon spokesman Bryan Whitman said.
It was unclear whether the pirates who seized the 530-foot-long cargo ship Faina on Thursday knew what it carried. Still, analysts said it would be extremely difficult to sell such high-profile weaponry like Russian tanks.
The hijacking, with worldwide pirate attacks surging this year, could help rally stronger international support behind France, which has pushed aggressively for decisive action against Somali pirates.
Russian navy spokesman Capt. Igor Dygalo told The Associated Press that the missile frigate Neustrashimy left the Baltic Sea port of Baltiisk a day before the hijacking to cooperate with other unspecified countries in anti-piracy efforts.
But he said the ship was then ordered directly to the Somalia coast after Thursday's attack.
According to the British-based Jane's Information Group, the Neustrashimy is armed with surface-to-air missiles, 100 mm guns and anti-submarine torpedoes.
Ukrainian Defense Minister Yury Yekhanurov, meanwhile, said the hijacked vessel Faina was carrying 33 Russian-built T-72 tanks and a substantial quantity of ammunition and spare parts. He said the tanks were sold to Kenya in accordance with international law.
Ukrainian officials and an anti-piracy watchdog said 21 crew members were aboard the seized ship, including three Russians. Ukrainian President Viktor Yushchenko ordered unspecified measures to free the crew, but it was unclear whether any of the former Soviet republic's naval vessels had been dispatched.
A Kenyan government spokesman, Alfred Mutua, confirmed the East African nation's military had ordered the tanks and spare parts. The tanks are part of a two-year rearmament program.
"The government is in contact with international maritime agencies and other security partners in an endeavor to secure the ship and cargo," Mutua said in a statement. "The government is actively monitoring the situation."
A person who answered the telephone at Ukrainian state-controlled arms dealer Ukrspetsexport, which brokered the sale, refused to comment, and said all requests for information must be submitted in writing.
It was unclear where the shipment originated, though Ukrainian news agencies identified the ship operator as a company called Tomex Team based in the Black Sea port of Odessa. Calls to Tomex offices went unanswered Friday.
At the Pentagon, two defense officials said the warship was tracking the Ukrainian ship but there has been no decision about taking any other action such as intercepting it. The officials said that besides the T-72 tanks, it was carrying guns and rocket launchers. The officials spoke on condition of anonymity because they were not authorized to discuss the matter publicly.
"Obviously, we are deeply concerned," said Lt. Nate Christensen, a spokesman for the Bahrain-based U.S. 5th Fleet, declining to provide details.
Whitman, the Pentagon spokesman, said the United States was worried about the cargo.
"A ship carrying cargo of that nature being hijacked off the coast of Somalia is something that should concern us, and it does concern us," he said.
The Navy says the 5th Fleet includes the USS Ronald Reagan aircraft carrier and several support ships, which "deter destabilizing activities and ensure a lawful maritime order in the Arabian (Persian) Gulf, Arabian Sea, Gulf of Oman and Gulf of Aden."
Paul Cornish, head of the international security program at the London-based think-tank Chatham House said the tanks would be difficult to sell on to a third party - private buyers or warlords, for example - because of the logistics involved with keeping them operational.
"It's not like (stealing) a container full of machine guns, where all you need is a tin of bicycle oil," he said.
Roger Middleton, another Chatham House researcher, said it was unlikely the pirates knew there were tanks aboard the Faina, and also said unloading the cargo would be difficult.
"Most of their attacks are based on opportunity. So if they see something that looks attackable and looks captureable, they'll attack it," he said.
Middleton said it was unclear how the pirates might react if confronted by military action, noting that they have fled from authorities in the past. On the other hand, he said, they are usually well-armed and organized and are based in an unstable country - Somalia.
"It could potentially get pretty messy," he said.
Long a hazard for maritime shippers - particularly in the Indian Ocean and its peripheries - high-seas piracy has triggered greater alarm since the Sept. 11 terrorist attacks on the United States because of its potential as a funding and supply source for global terrorism.
Pirate attacks worldwide have surged this year and Africa remains the world's top piracy hotspot, with 24 reported attacks in Somalia and 18 in Nigeria this year, according to the International Maritime Bureau's piracy reporting center.
The issue burst into international view Sept. 15 when Somali pirates took two French citizens captive aboard a luxury yacht and helicopter-borne French commandos then swooped in to rescue them.
French President Nicolas Sarkozy this month called on other nations to move boldly against pirates, calling the phenomenon "a genuine industry of crime."
In June, the U.N. Security Council - pushed by France and the United States - unanimously adopted a resolution allowing ships of foreign nations that cooperate with the Somali government to enter their territorial waters "for the purpose of repressing acts of piracy and armed robbery at sea."