Pirates seized the vessel, believed to be a commercial fishing trawler or a small freighter, sometime this week off the southeastern port town of Kismayo and are holding its crew hostage, the Juba Valley Alliance (JVA) militia said.
"Some gunmen, who are really freelancers, are holding it at Kiyoma island," a spokesman for the JVA, Abullahi Sheikh Ismail, told AFP by phone from Kismayo, about 500km south of Mogadishu.
"At this time, I'm not sure of the number of crew who are hostage," he said. "I don't know their nationality but they look like they are Chinese or related people from Indochina."
Ismail said the ship had been in Somali waters illegally, but stressed that the JVA, which controls the area of lawless Somalia where the hijacking took place, did not believe its seizure was justified.
Details of the incident were sketchy, but unconfirmed reports reaching Mogadishu on Wednesday said the vessel was Chinese-owned and that it may have been under contract to an international relief group when it was boarded.
A UN-chartered ship, the MV Semlow, which was carrying food aid to Somali tsunami victims, has been held by pirates along with its crew and cargo for nearly two months further north along the coast.
"Some gunmen, who are really freelancers, are holding it at Kiyoma island"
The new hijacking was reported just days after the International Maritime Board (IMB) renewed its warning for vessels to avoid the coast of Somalia, citing a recent "alarming" surge in the number of attacks.
"The threat posed to vessels operating off the eastern Somali coast is very real and should not be understated," it said in a statement on Monday, adding that "acts of piracy are increasing at an alarming rate."
At least 15 violent incidents, including the hijacking of the Semlow, have occurred since mid-March, compared with just two in 2004, it said.
On Tuesday, in its weekly international piracy report, the IMB said nine of those incidents had been reported since 16 June, many of which involved pirates opening fire on vessels with automatic weapons.
The board has been warning ships since June to stay at least 93km, and preferably further, away from the Somali coast.