Mwangura identified the ship as North Korean, and said it was possibly involved in a business dispute. Another source said the ship was carrying sugar from Brazil.
On Sunday, pirates had hijacked a Japanese-owned chemical tanker flying the Panamanian flag off Somalia with 23 people on board.
The US Navy on Tuesday said coalition naval forces belonging to Combined Task Force 150 had pursued the pirates into Somali waters and opened fire, destroying speedboats the seized vessel had in tow that were used in the raid.
"CTF-150 responded to a distress call from the tanker Golden Nory, warning shots were fired and the skiffs in tow were engaged and sunk," a spokesman for the US Navy's Fifth Fleet said.
There were no reports of any casualties. He said coalition forces had opened fire in the Gulf of Aden.
"The operation is ongoing [to recover the tanker] and there are indications a number of pirates are still on board," the spokesman said, adding that a number of battleships were in the area.
Mwangura said the Golden Nory was carrying benzene, an inflammable and toxic chemical, and was being held off the northern Somali province of Puntland.
Four other boats - a Comoros-registered cargo ship, two Tanzanian fishing vessels, and a ship from Taiwan - are also being held by armed groups.
Without central government since 1991, Somalia's waters have become among the world's most perilous despite calls for international action to patrol them.
Attackers often justify their actions as measures against illegal fishing and toxic dumping.
Mwangura said the recent increase in attacks off Somalia may be a "message of defiance" in response to threats by the UN Security Council and others to take action against piracy.