US warships are heading to the area following Capt Phillips' capture
Somali pirates have hijacked a tugboat in the Gulf of Aden with 16 crew members on board - 10 of them Italians.
Maritime industry sources say the tug was towing two barges at the time of the attack at 0800 GMT. The crew are said to be unharmed.
Meanwhile pirates holding a US captain hostage have warned that using force to rescue him could result in "disaster".
They said they hoped to put Capt Richard Phillips on a larger vessel. US ships are on their way to the area.
He is being held by four pirates in a lifeboat hundreds of kilometres off Somalia.
In other developments:
- Sailors on a Panama-flagged bulk carrier repulsed a pirate attack with water hoses. Nato officials on a nearby Portuguese warship said an unexploded rocket-propelled grenade landed in the commanding officer's cabin
- On Friday, one Frenchman and two pirates were killed in a rescue operation by French troops on another vessel captured off Somalia. Four others, including a child, were freed from the yacht
Earlier reports suggested that a group of Somali elders were preparing to mediate between American officials and the pirates for Capt Phillips' release, but there has been no news of any progress.
The US national was taken hostage on Wednesday after pirates hijacked his ship, the Maersk Alabama, as it sailed towards the Kenyan port of Mombasa carrying food aid.
MAJOR PIRATE INCIDENTS
Ukrainian ship MV Faina seized on 25 September 2008, held until 5 February 2009
Saudi tanker Sirius Star held for two months from November 2008; a $3m ransom was negotiated
At least 15 pirate attacks reported to International Maritime Bureau during March 2009
The 32,500-tonne Malaspina Castle, UK-owned but operated by Italians, seized on 6 April 2009.
After a long struggle, crew members regained control of the ship. It is thought Capt Phillips offered himself as a hostage in order to save his crew.
The Maersk Alabama arrived in Mombasa late on Saturday.
"He's a hero," one crew member shouted as the ship moored, referring to Capt Phillips.
The BBC's Karen Allen at the port says there is unprecedented security.
FBI agents are on board the ship.
Maersk President and Chief Executive Officer John Reinhart told journalists the ship was now a crime scene and the crew would have to stay on it during the investigation.
They would all be repatriated as soon as possible, he added.
There has been rising concern in the US over the fate of the captain - Defence Secretary Robert Gates told reporters in Washington on Friday that his safe return was a "top priority".
FBI experts are helping negotiate his release, but analysts have said the process could be lengthy.
Capt Phillips tried to escape on Friday by jumping overboard and swimming towards a nearby US ship, but was recaptured.
Reports in the US say the pirates are demanding a ransom of $2m for his safe release.
The US Navy destroyer, USS Bainbridge, is at the scene of the confrontation with other American warships on the way.
Stories circulating in Somalia claim that extra pirate ships are also making their way towards the area.
The pirates say they hope to transfer Capt Phillips to a bigger and better-protected vessel.
The Pentagon is also said to be considering other options, including possible military force, US sources say.
But the Somali pirate commander warned against any forcible intervention.
"I'm afraid this matter is likely to create disaster because it is taking too long and we are getting information that the Americans are planning rescue tricks like the French commandos did," Abdi Garad said.
The attacks have renewed international focus on hijackings in one of the world's busiest shipping lanes.
Florent Lemacon, the owner of the French yacht and father of the child who was on board, was killed during the rescue operation.
French Defence Minister Herve Morin said on Saturday that officials "cannot rule out" that Mr Lemacon was killed by French fire.
But he said the raid was "the best possible decision," and that an investigation would determine what happened on board the Tanit.
The four released hostages - Mr Lemacon's wife Chloe, their three-year-old son Colin, and two other adults - are due to arrive in Paris on Sunday, he said.
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.