NAIROBI, Kenya Pirates in a hijacked ship carrying more than 30 battle tanks were anchored off the coast of Somalia near a notorious pirate den on Saturday, and they vowed not to release the ship until a $35 million ransom was paid, Somali and Kenyan officials said.

Meanwhile, a United States Navy destroyer, the Howard, was closing in on the hijacked ship.

“We’ve got a constant stare on it,” said Rear Adm. Gregory Smith, an American military spokesman. He would not say how far away the destroyer was from the hijacked vessel, but he acknowledged that the American ship was close enough to maintain visual contact. He also said there were no imminent operations planned against the hijacked ship.

According to Mohamed Osman Aden, a Somali diplomat in Kenya, the hijacked ship, a Ukrainian-owned vessel, was headed on Saturday afternoon to Xarardheere, on the barren Somali coast. Xarardheere is an isolated fishing village that has thrived on organized crime and has frequently been used as a pirate hide-out and a place to keep seized ships.

Mr. Mohamed said that while the cargo in this case was extremely unusual  33 Soviet-designed T-72 tanks and a large supply of ammunition and grenade launchers, all intended for the Kenyan military  the tactics were pretty typical.