Residents near the ship, which was hijacked on Monday with eight Sri Lankan crew aboard, confirmed hearing the gunfire.
The director of Puntland's anti-piracy agency, Abdirizak Mohamed Ahmed, said they were not aware of any threats issued by the pirates against the crew. "They made (the pirates) an offer they couldn't refuse and the pirates have left", said John Steed, a former British army officer with the NGO who has spent years negotiating the release of piracy hostages in Somalia.
Naval forces and the pirates clashed earlier on Thursday.
The hijacking was the first time Somali pirates had taken over a commercial ship since 2012 and followed an outpouring of anger by locals over foreign fishermen flooding into their waters.
However, the local manpower agent who recruited the vessel crew Tyron Fernando said during a media conference held at the Ministry of Shipping that the pirates have not demanded any ransom as yet.
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The tanker was forced to change course and head toward Puntland.
The gang said they agreed to forego a ransom after finding out that Somali businessmen had hired the tanker.
Steed says the global community had taken significant steps to improve security, boosting naval forces in the area and requiring ships to take protection measures.
Aircraft from regional naval force EU Navfor were flying overhead to track the ship's progress and to try to determine what was happening, he said.
The tanker, which was en route from Djibouti to the Somali capital, Mogadishu, was seized on Monday with eight Sri Lankan crew members on board.
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"If they do not get off, we shall fight to rescue the ship", Hassan told Reuters.
The ship was anchored off the town of Alula, said Salad Nur, a local elder.
Pirates have traditionally been wary of getting caught up with the country's powerful businessmen.
"These are fishermen who are infuriated with the illegal fishing off their coasts".
A United Nations report seen by the AP in November said Somali pirates retain the capacity and intent to resume the attacks and lately have shifted to targeting smaller foreign fishing boats.
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Pirates argued that they are fighting illegal ships and boats which are collecting their natural resources, mainly the fish.