Pirates seized the MV Faina off the Somali coast last week and have demanded $20 million in ransom.
U.S. navy ships are within sight of the boat, whose capture has sparked controversy over the destination of its military cargo and thrown an international spotlight on rampant piracy in one of the world`s busiest shipping areas.
Andrew Mwangura, of the East African Seafarers` Assistance Programme, said rival factions among the roughly 50 pirates guarding the Faina had argued over whether to give in to international pressure to free the cargo and 20-man crew.
"There was a misunderstanding yesterday between the moderates and the radicals on board who do not want to listen to anyone," said Mwangura, whose Kenya-based group is monitoring the saga via relatives of the crew and the pirates.
"The moderates want to back-peddle. The Americans are close, so everyone is tense. There was a shootout and three of the pirates were shot dead."
The U.S. navy has said the ship, which was heading for Kenya`s Mombasa port, was carrying T-72 tanks, grenade-launchers and ammunition ultimately bound for south Sudan via Kenya.
A fragile peace has held in south Sudan since 2005 after more than two decades of war with the north. A major arms shipment could violate the terms of that pact unless it was specifically authorised by a north-south committee.
But Kenya says the armoury was for its military. "It is the property of the Government of Kenya and we have documentation to that effect," said military spokesman Bogita Ongeri.
In Kenya, civil groups have demanded the government to explain why it would spend so much on military equipment when it is struggling to help refugees from post-election violence.
Taking advantage of chaos on shore in Somalia, where an Islamist-led insurgency has been raging for nearly two years, pirates have hijacked more than 30 ships this year and attacked many more.
Most attacks have been in the Gulf of Aden between Yemen and north Somalia, a major global sea artery used by about 20,000 vessels a year heading to and from the Suez. The pirates have also struck in the busy Indian Ocean waters off south Somalia.
With U.S. and French military bases in the area, and the U.N. Security Council having promised to take steps against the pirates, many are unhappy with the lack of international action.
"If civil aircraft were being hijacked on a daily basis, the response of governments would be very different," top shipping trade bodies and transport unions said in a joint statement on Monday.
"Yet ships, which are the lifeblood of the global economy, are seemingly out of sight and out of mind," the groups added. More than 90 percent of the world`s traded goods by volume are carried by sea.
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