The cruise-missile carrying USS Bainbridge was sent to the scene in a move analysts say will strengthen the hand of US negotiators.
The destroyer is shadowing the lifeboat, and its officers are talking to the pirates, as well as giving them food and batteries for a two-way radio.
Capt Phillips has a radio and contacted the navy and crew of his ship to say he was unharmed, the Maersk shipping company said in a statement.
FBI experts are helping negotiate his release.
Analysts have said negotiations could be lengthy, with the pirates likely to want a hefty ransom for the captain as well as compensation for a boat that was wrecked in the attack.
Mohamed Samaw, a Somali who claimed to have a "share" in a British ship hijacked this week, told AP news agency two hijacked ships had left the pirate stronghold of Eyl on Wednesday afternoon.
The USS Bainbridge remains close to the scene of the stand-off
He said a further two seized vessels were also sailing towards the lifeboat.
Two of the ships have some 54 hostages aboard, from China, Germany, Russia, Ukraine, the Philippines, Tuvalu, Indonesia and Taiwan.
A man identified as a pirate named Badow told AP: "They had asked us for reinforcement and we have already sent a good number of well-equipped colleagues, who were holding a German cargo ship."
He said there was no intention of harming the captain, as long as the pirates holding him were not hurt.
"All we need, first, is a safe route to escape with the captain, and then (negotiate) ransom later," he said.
The cargo ship, carrying food aid destined for Somalia and Uganda, was seized about 500km (311 miles) off Somalia's coast in the early hours of Wednesday.
Capt Phillips' sister-in-law Lea Coggino says he is "a smart guy who is in control"
After a long struggle, the crew members regained control of the ship.
It is thought that Capt Phillips offered himself as a hostage in order to save his crew.
Zoya Quinn, wife of the ship's second officer, told the Associated Press that Capt Phillips had told the crew to lock themselves in a room and that the pirates, who were "desperate", searched all over the ship for them.
The crew held a wounded pirate for about 12 hours, dressing his wounds "because he was bleeding all over the ship", she said, after communication with her husband Ken by phone and e-mail.
Somalia has been without an effective government since 1991, fuelling the lawlessness which has allowed the pirates to thrive.
Pirates typically hold the ships and crews until large ransoms are paid by the shipping companies - last year the firms handed over about $80m (£54m).
Efforts to stop the pirates have so far had only limited success, with international naval patrols struggling to cover the vast areas of ocean where pirates operate.
The UN's Somalia envoy, Ahmedou Ould-Abdallah, told Reuters that piracy was threatening to destabilise the region.
In what may prove an important test case, Kenyan officials are currently seeking to try seven pirates captured by US forces in February, the Wall Street Journal reports.
It has previously proved difficult to prove national jurisdiction because pirate attacks usually take place in international waters.
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