HTAN KONE, Myanmar (Reuters) - Myanmar authorities detained a dozen rioters on Sunday a day after clashes between Buddhists and minority Muslims in the northern region of Sagaing, police said, in the latest widening of sectarian violence in the former military-run state.

Witnesses said between 500 and 1,000 Buddhists, some carrying sticks and swords, attacked Muslim houses in Htan Kone village on Saturday, setting some buildings on fire.

Police and soldiers arrived in the evening and fired into the air to disperse the crowds, they said.

The violence in the rugged region about 665 km (410 miles) from the commercial capital, Yangon, shows how far anti-Muslim anger has spread in the Buddhist-dominated country following spasms of unrest in northeastern Lashio in May, central Meikhtila in March and western Rakhine State last year.

"We have arrested altogether 12 people from the mob and they were transferred to a district station," police sergeant Win Nyi told Reuters.

Some Muslims fled to neighboring villages or sheltered in a Muslim school, residents of the area said.

Houses were already burning by the time police and soldiers arrived, said Saya Soe, 32, a Muslim villager. "The mobs stopped and went away only after midnight when the security forces fired four or five shots into the sky."

Another Muslim resident, Azit Paing, said the cause of the unrest appears to have been an argument between a young Muslim man and a Buddhist woman. Myanmar's Buddhist-dominated media reported rumors that she had been raped. Muslim villagers denied this.

Clashes between Buddhists and Muslims have threatened to undermine political and economic reforms the government initiated two years ago after decades of military misrule.

More than 200 people have been killed since June last year and 140,000 displaced. The vast majority of victims have been Muslim.

Tomás Ojea Quintana, a U.N. special rapporteur on human rights, said a 200-strong mob attacked his car during an August 22 visit to the central Myanmar town of Meikhtila, where a wave of anti-Muslim riots in March killed at least 43 people, destroyed hundreds of homes and displaced thousands.

Presidential spokesman Ye Htut denied Ojea Quintana had been attacked and said the United Nations mistook the crowd's intentions.

(Reporting by Min Zayer Oo and Jared Ferrie; Editing by Jason Szep and Robert Birsel)