An American warship, the USS Bainbridge, has reached the area off the coast of Somalia where a cargo ship was seized by pirates a day earlier.
US crew members have recaptured their ship but the captain is still being held hostage by the attackers.
The pirates' boat is floating near the Maersk Alabama, its owners told the Associated Press. Officials are waiting for sunrise to see what happens.
The ship was taken about 500km (311 miles) off Somalia's coast.
Other US vessels are speeding towards the scene.
US Secretary of State Hillary Clinton said the government was following the situation very closely and urged the world to act to end the "scourge" of piracy. BBC NEWS | Africa | US warship reaches Somali coast