Baghdad: The Islamic State group has carried out a fresh wave of mass killings, officials said Sunday, executing more than 200 members of an Iraqi tribe, which took up arms against the jihadists. Women and children were said to be among scores of Albu Nimr tribes people executed over the past 10 days in western Iraq's Anbar province.

Reports of the killings came with the country on edge as hundreds of thousands of Shiites prepare to travel to shrine city Karbala this week for a major annual pilgrimage. IS, a Sunni extremist group that has seized large parts of Iraq and Syria, is expected to target Ashura pilgrims, and 13 people died in an attack on Shiites on Sunday.

The executions in Anbar came after Sunni Albu Nimr tribesmen took up arms against IS in the province, large parts of which have been overrun by the jihadists. Accounts varied as to the number and timings of the executions, but all sources spoke of more than 200 people murdered in recent days.

Police Colonel Shaaban al-Obaidi told AFP that more than 200 people were killed, while Faleh al-Essawi, deputy head of Anbar provincial council, put the toll at 258. The killings are probably aimed at discouraging resistance from powerful local tribes in Anbar.

IS also detained dozens of members of the Jubur tribe in Salaheddin province, north of Baghdad, officials and a tribal leader said. Jubur tribesmen and security forces have been holding out for months against IS in the provincial town of Dhuluiyah.

Pro-government forces have suffered a string of setbacks in Anbar in recent weeks, prompting warnings that the province, which stretches from the borders with Jordan and Saudi Arabia to the western approach to Baghdad, could fall entirely.

Security forces who wilted before a lightning IS offensive in June are fighting to retake territory seized by the jihadists in Iraq's Sunni Arab heartland. IS has declared an Islamic "caliphate" in territory it controls, imposing its harsh interpretation of sharia law and committing widespread atrocities.

Like other Sunni extremist groups, IS considers Shiites to be heretics and frequently attacks them, posing a major threat to the Ashura religious commemorations which peak on Tuesday. A car bomb targeting Shiites in Baghdad ahead of Ashura killed at least 13 people on Sunday, officials said, while a city centre car bombing near a police checkpoint killed at least five.

The pilgrimage is a major test for the new government headed by Prime Minister Haidar al-Abadi and for the security forces. On the Syria-Turkey border, meanwhile, some 150 Iraqi peshmerga fighters were preparing to bolster fellow Kurds in battling IS for the town of Kobane, after crossing the frontier late on Friday.

Syrian Kurdish militia have held off an IS offensive there for more than six weeks, and Kobane has become a crucial symbol in the anti-jihadist struggle. The Syrian Observatory for Human Rights, a Britain-based monitor, reported fierce clashes in the town's centre, north, south and Kurdish fighters shelling IS positions to its east.

The US-led coalition that has launched air raids against IS in Syria and Iraq carried out at least three strikes near Kobane early Sunday, said the Observatory, which relies on a wide network of sources inside the country. At least 11 jihadists were killed in air strikes and fighting on Saturday, it said.

The Pentagon said five air strikes near Kobane on Saturday and Sunday hit five small IS units and destroyed three vehicles. In Iraq, coalition warplanes targeted IS positions in Baiji and Fallujah, it said. 

Elsewhere in Syria, the Observatory said Al-Qaeda affiliate Al-Nusra Front seized a town and several villages in Idlib province late Saturday, in another blow to Western-backed rebels in the northwest.

It said Al-Nusra captured Khan al-Subul after the withdrawal of the Hazm movement, a moderate opposition group. Al-Nusra also seized another five villages in Idlib held by Islamist and moderate rebel groups. The advance comes a day after Al-Nusra seized the Idlib bastion of the Syria Revolutionaries Front, another Western-backed opposition group.

The advance of the Al-Qaeda affiliate is seen as a setback to US efforts to create and train a moderate rebel force as a counterweight to jihadists and the forces of Syrian President Bashar al-Assad.