* Defence ministry says troops came under fire from gunmen
* Protesters say unarmed when security forces opened fire (Adds details from ministry)
By Suadad al-Salhy
BAGHDAD, April 23 (Reuters) - At least 26 people were killed when Iraqi security forces stormed a Sunni Muslim protest camp near Kirkuk on Tuesday, sparking a gun battle between troops and protesters that threatens to inflame sectarian tensions.
The clashes were the bloodiest since thousands of Sunni Muslims began staging protests in December to demand an end to perceived marginalisation of their sect by Prime Minister Nuri al-Maliki's Shi'ite-led government.
Iraq's defence ministry said troops responded only after coming under fire from gunmen in the makeshift camp in a public square in Hawija, near Kirkuk, 170 km (100 miles) north of the capital Baghdad.
"When the armed forces started ... to enforce the law using units of riot control forces they were confronted with heavy fire," the defence ministry said in a statement.
But protest leaders said they were unarmed when security forces stormed in and started shooting during the early morning raid on the camp.
Demonstrators and local officials gave conflicting accounts of the number of casualties, but the defence ministry said 20 people at the camp and three officers had been killed.
At least three military sources put the toll at six troops and 20 demonstrators killed.
"When special forces raided the square, we were not prepared and we had no weapons, they crushed some of us in their vehicles," said Ahmed Hawija, a student who had been taking part in the demonstrations.
Since the last U.S. troops left in December 2011, Iraq's government has been mired in crisis over how to share power among the Shi'ite, Sunni and ethnic Kurdish parties. Maliki's critics accuse him of amassing power at their expense.
Many Iraqi Sunnis say they have been sidelined after the U.S.-led 2003 invasion that ousted Sunni strongman Saddam Hussein and allowed the Shi'ite majority to gain power through elections.
Sunni protests erupted in December after security forces arrested the bodyguards of the country's Sunni finance minister as part of a counter-terrorism operation, which Sunni leaders dismissed as part of a political crackdown on Maliki's foes.
Maliki tried to ease protests by offering some concessions on reforming tough anti-terrorism laws and a law targeting former members of Saddam's outlawed Baathist party - both of which Sunnis fear were used unfairly to target them.
But Sunni protest leaders and politicians are also deeply divided over how to manage relations with the government, with moderates working alongside Maliki and more hardline Sunni Islamist leaders demanding the Shi'ite leader step down.
Security officials say country's al Qaeda wing has also sought to tap into Iraqi Sunni frustrations. Al Qaeda's affiliate, Islamic State in Iraq, has stepped up attacks this year to try to stir widespread confrontation between Iraq's Sunni and Shi'ite Muslims. (Additional reporting by Kareem Raheem in Baghdad and Gazwan Hassan in Samarra; Writing by Patrick Markey; Editing by Catherine Evans)