MOSCOW—A Russian warship on Friday rushed to intercept a Ukrainian vessel carrying 33 battle tanks and a hoard of ammunition that was seized by pirates off the Horn of Africa—a bold hijacking that again heightened fears about surging piracy and high-seas terrorism.

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.