Somali pirates who seized an oil tanker have opened fire on naval troops from the semi-autonomous state of Puntland, sparking clashes between the two sides, an official said.
Clashes started after naval forces tried to stop a boat carrying reinforcement pirates to the ship, said Ali Shire Mohamud, the commissioner of Alula district where the vessel is being held.
Puntland dispatched naval forces to the area after the pirates seized the ship on Monday.
The European Union anti-piracy operation in the region said the pirates are holding the crew captive and demanding a ransom.
The hijacking on Monday was the first such seizure of a large commercial vessel off Somalia since 2012.
International anti-piracy patrols on the crucial trade route had calmed such attacks.
A security official said one soldier was seriously injured in Thursday's clashes involving the Comoros-flagged tanker Aris 13.
The official said pirates on the ship were continuing to receive reinforcements while regional forces mobilise nearby in an effort to free the ship.
A Somali pirate who said he was in touch with the armed men, Bile Hussein, said the pirates moved the ship away from the coast after naval forces approached.
"The ship and crew will remain safe as long as no-one attacks them," Hussein said by phone from an undisclosed location in northern Somalia.
Families of the crew members tearfully pleaded on Wednesday for the men to be released unharmed.
Somali pirates usually hijack ships and crew for ransom. They do not normally kill hostages unless they come under attack.
The ship had been carrying fuel from Djibouti to Somalia's capital, Mogadishu, on Monday when it was approached by men in two skiffs and the tanker issued a call for help.