Survivors of the Oman mosque attack have told of the moment gunmen opened fire with automatic weapons as they shouted: “You non-believers, this is your end.”

Worshippers thought the pops and cracks were firecrackers as they gathered at Imam Ali Mosque to mark an occasion sacred to Shiite Muslims, until bodies began falling to the ground.

“We fell to the ground … bullets hit the wall and road all around us,” Shaandar Bukhari, a mosque volunteer from Attock, Pakistan, told The National.

“The sniper was chanting loudly as he shot at us. It was in pure Arabic – not someone who would have learnt Arabic – and he shouted: “You non-believers, this is your end.'”

ISIS has claimed responsibility for the attack on Monday night, which left nine people dead, including three gunmen, and dozens injured.

It broadcast a video showing three men holding the black ISIS flag before the attack in Wadi Al Kabir, a town outside Muscat. One can be seen holding an Austrian-made Steyr assault rifle. Their identities are not yet known.

The gunmen left dozens wounded and went on to trade fire with Omani police and troops until they were gunned down. The siege began at 10.15pm and lasted for more than 10 hours, ending on Tuesday.

Worshippers were shepherded out in batches from 11.30pm on Monday until 5am on Tuesday.

Omani authorities have released few details about the incident. It is not clear how the gunmen sourced weapons in a country with low gun ownership. ISIS has previously claimed responsibility for atrocities it had no direct hand in.

Streets in Wadi Al Kabir remained blocked off as the investigation continued on Wednesday.

The Omani government said it was ready to "confront any challenges" to its national security, in the wake of the shooting and the capsizing of an oil tanker off the coast of Oman.

“Our dear land witnessed an exceptional day when it was exposed to two unprecedented security incidents, and our security services responded to them with great efficiency, demonstrating to the entire world their readiness and ability to confront any challenges that arise,” the government said in a statement.

The statement said investigations are ongoing to determine the motive behind the incidents.

'Terrorist attack'

Worshippers said the gunmen were standing on the roof of an adjacent building and used the floodlights there to view targets in the courtyard below.

Mr Bukhari, 42, who heads a team of mosque volunteers and works for an advertising firm in Muscat, said he could not forget the violent images.

He crawled to the car park near the women’s section to carry two crying children to safety, but their father died in his arms.

“I saw a man in a white T-shirt holding an automatic rifle. He was looking away from me and then I got a chill down my spine when I saw two kids taking shelter between a car and the wall of the mosque,” Mr Bukhari told The National from the hospital where he is being treated after bullets grazed his arms.

“They were sitting on the body of their father and crying, ‘We will not leave our baba.’

“I picked them up and literally threw them into the mosque. Their father was badly wounded. [He] did not think he would make it and kept asking us to save his children.

“With another volunteer, we lifted him to take him in but he was shot in a burst of bullets, he died in our arms.”

Mr Bukhari spoke of the horror of hiding under bodies to escape the barrage of bullets.

“People were shot in the back and legs in front of my eyes as we crawled to the gate,” he said.

“I was lying under a body with two bodies near me. There was loud, constant firing.”

Like others, Mr Bukhari called the authorities by phone.

“We called to ask people to save us, we said there is a terrorist attack on our mosque,” he said.

“These men were barbaric, inhuman. If the Oman police had not come quickly, many more would have died.”

Raining bullets

Mr Bukhari said he spotted at least four gunmen on the roofs of buildings overlooking the mosque during the “relentless” attack.

A small alley between the men's and women’s section of the mosque was also attacked by the gunmen.

“They were firing on us from behind floodlights so we could not see them but they could see us clearly,” Mr Bukhari said.

“When people fell down, they were still firing at the bodies as if to make sure they wouldn’t survive.

“Then I heard bursts of fire that were definitely automatic machinegun bursts.

“It was raining bullets around me and people were falling near me.

“We began pushing people into the mosque.

“But the bullets started hitting the gate, the bullets were so strong, it pierced the gate, the walls.”

'Escaping death'

In the men’s section of the mosque, Syed Hassan, a businessman from Lahore, appealed to worshippers to stay calm.

“We tried to keep people calm but there was so much fear because there was so much shooting,” said Mr Hassan.

He and his teenage son narrowly escaped as they ran for shelter.

“We pushed people inside and shut the doors so they could not enter,” he said.

“This saved lives. But when we were running, my son was hit by a bullet in his right thigh. A bullet went right by my neck and into the door.”

Mr Hassan quickly turned off the lights in the women’s section so the gunmen would not have a clear sight inside.

While tending to his son and others injured, Mr Hassan said he worried about his wife and three children in the women’s section.

He managed to contact the women's section to pass on a message from the police to remain inside.

He and other volunteers have been monitoring the crowds at the mosque for more than a decade.

“If we had not pulled people inside and they were still standing on the road, there would have been a lot more casualties,” he said.

“If not for the local authorities and police many more would have died.”

Additional reporting by Khaled Moussa in Muscat.