COLOMBO - Sri Lanka's army said on Wednesday it was investigating a video posted on social media that showed a man dressed as a soldier walking away seconds before a mob attacked a building during anti-Muslim riots this week.

Mobs moved through towns in Sri Lanka's northwest, ransacking mosques, burning Korans and attacking shops with petrol bombs, residents said, in apparent reprisal for Easter bombings by Islamist militants.

The April 21 attacks, claimed by Islamic State, targeted churches and hotels, killing more than 250 people and fuelling fears of a backlash against the island nation's minority Muslims.

In the video, a man wearing what appeared to be an army uniform stands outside a building and then walks away. Seconds later, about two dozen people, including young men wearing motorbike helmets, run over and throw stones at the building.

The video could not be independently verified by Reuters.

"The attention of the army has been drawn to a video clip where a person dressed in uniform similar to that of the army was watching while a group of violent saboteurs were in action in the general area of Thunmodara," the army said in a statement announcing the investigation.

Two residents of Thunmodara told Reuters that a mosque and some Muslim-owned shops were attacked in the town in Kurunegala district that was hardest hit by the rioters.

Authorities said they have arrested more than 74 suspects in connection with the anti-Muslim attacks, including three Sinhala Buddhist extremists who had been investigated for similar actions in Kandy last year.

In recent years, Buddhist hardliners, led by the Bodu Bala Sena (BBS) or "Buddhist Power Force" have stoked hostility against Islam, saying Middle Eastern influence has turned Sri Lankan Muslims more conservative and isolated.

In over a dozen interviews, Muslims in Kurunegala said the attacks took place despite the presence of security forces.

One police source who declined to be identified told Reuters they did not have enough officers to handle the rioters.

Police spokesman Ruwan Gunasekera on Tuesday rejected allegations that police had stood by while the violence unfolded and said the perpetrators could face up to 10 years in jail.

(Additional reporting by Alexandra Ulmer Editing by Alexandra Ulmer and Darren Schuettler) ((shihar.aneez@thomsonreuters.com; +94-11-232-5540; Reuters Messaging: shihar.aneez.thomsonreuters.com@reuters.net twitter:@shiharaneez))