The hijacking is the latest in a series of raids in the region. At least a dozen ships have been seized off the Somali coast, with more than 200 crew members being held hostage.
Somali pirates have continued to operate despite the presence of an international task force gathered to defend shipping in what is one of the world's busiest shipping lanes.
Captain's fate unknown
The capture of the tugboat comes a day after another group of Somali pirates recaptured Richard Phillips, an American sailor, when he tried to escape from the lifeboat in which he is being held hostage.
Phillips had jumped into the water on Friday and tried to swim towards the USS Bainbridge, a nearby destroyer.
|Phillips was being held hostage in a lifeboat as his ship arrived in Kenya on Saturday [Reuters]
He was seized off the coast of Somalia after an abortive attempt on Wednesday by the pirates to hijack his ship, the Maersk Alabama
, which was carrying 5,000 tonnes of UN aid for African refugees.
The vessel arrived in the Kenyan port of Mombasa at 17:30 GMT on Saturday without Phillips.
Abdi Gara, a pirate commander, said on Friday that Phillips would be moved from the lifeboat to another ship off the Somali coast.
Only four pirates are guarding him on the lifeboat, but transfer to a larger ship would make it easier for them to thwart any US military attempt to free him.
US navy forces have been pouring into the region amid the standoff over Phillips.
Garad warned that any attempt to free Phillips would be disastrous.
"I'm afraid this matter is likely to create disaster because it's taking too long and we are getting information that the Americans are planning rescue tricks like the French commandos did," he said.
Garad was referring to storming on Friday by French marine commandos of the Tanit, a yacht, in which Florent Lemacon, a French yachtsman, and two pirates died.
The pirates were taking the Tanit in towards the Somali coast and had refused offers of a ransom, a spokesman for French President Nicolas Sarkozy said.
Four hostages, including Lemacon's three-year-old son, walked free and three pirates were captured as a result of the raid.
Mohamud Muse Hirsi, the president of Somalia's autonomous Puntland region, said giving in to the pirates' demands is not an option.
"We do not advocate for any ransom to be paid to the pirates and we support the French government, which uses force, while taking on the pirates," he said.
Puntland is reputed to be the home of many pirates.
Vessels captured by Somali pirates in the Gulf of Aden are said to fetch on average a ransom of $2 million.
Ali Abdullahi, a security consultant and risk management analyst specialising on Somalia, said the only way to tackle piracy is to address the political and economic problems there.
"A lot of the time, we're not talking about the core issues which have made piracy a major event in Somalia," he told Al Jazeera.
"There has been a lot of illegal fishing by international agents, toxic dump wasting on parts of Somalia, a lack of bad governance as well, all of which are the real causes of piracy."