CNN) -- Gunfire broke out Tuesday aboard a cargo ship carrying tanks and other heavy weapons that was hijacked last week by pirates off the coast of Somalia, a Kenyan maritime official says.
A photo from the USS Howard shows Somali pirates in small boats hijacking the MV Faina last week.
It was not known whether the gunfire was among the pirates or an exchange of shots with authorities near the vessel, said Andrew Mwangura of the Kenya Seafarers Association.
On Monday, a shootout between rival pirate factions left three pirates dead, he said.
That shootout centered on a disagreement between moderate and radical pirates aboard the MV Faina, Mwangura said. The moderates wanted to surrender, but the radicals did not.
The presence of the U.S. Navy nearby had made some of the pirates nervous, Mwangura said.
Several Navy ships continued to monitor the seized ship, which is loaded with Soviet-era tanks and weapons, Lt. Nathan Christensen said Tuesday.
Christensen declined to specify how many U.S. ships were involved. He also declined to specify how close the warships were to the seized vessel other than to say, "Several are in visual range, and several are in the immediate vicinity."
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The pirated ship, he said, was anchored off the coast inside Somalia's territorial 12-mile limit.
The Navy ships were not involved in negotiations between the pirates and the shipping company, Christensen said.
"We want the crew to remain safe and for the cargo not to fall into the wrong hands," Christensen said.
The pirates, who Christensen said were "loitering on and around the ship," had lowered their ransom demand from $35 million to $20 million.
At the Pentagon, spokesman Geoff Morrell said U.S. officials are worried about who the pirates could sell the weapons to.
"Will it end up in the hands of responsible entities, such as the government of Kenya, who will deal with it appropriately?" Morrell asked. "Or will it end up in the hands of terrorists?" Watch how experts believe piracy could aid terrorists »
Mwangura said Monday that the organization's sources in Somalia reported the change in the ransom amount.
The pirates hijacked the ship off the coast of Somalia on Thursday. The Faina had been headed to the Kenyan port of Mombasa after departing from Nikolayev, Ukraine, and was seized not far from its destination.
The Faina is owned and operated by Kaalbye Shipping Ukraine, and its crew includes citizens of Ukraine, Russia and Latvia, the Navy said.
Abdi Salan Khalif, commissioner of the coastal town of Harardhere, said the pirates told a group of town elders that one crew member had died of high blood pressure problems.
The Ukrainian ministry said the ship is carrying 33 Soviet-made T-72 tanks, tank artillery shells, grenade launchers and small arms. Ukraine sold the weapons to Kenya, said Ukraine Defense Minister Yuri Yekhanurov, according to the Interfax-Ukraine news agency.