Government-aligned security forces in Iraq have attacked Sunni militant positions, as the insurgents' advance towards the capital Baghdad appeared to slow.

Iraqi security forces killed 279 militants in 24 hours and destroyed 50 of their vehicles, army spokesman Qassem Atta told a televised press conference.

"The security situation is improving," he said.

Government-aligned security forces launched a defensive operation in the crucial territorial belt surrounding the capital after Sunni militants, led by the ultra-violent Isis (Islamic State of Iraq and the Levant) swept to within striking distance of Baghdad.

Security sources told Reuters that Iraqi troops drove Isis militants out of al-Mutasim town, close to Samarra, while also taking control of the nearby town of Ishaqi.

Meanwhile, troops aligned with the Shia Asaib Ahl al-Haz militia helped push Isis from the town of Muqdadiya, north-east of Baghdad.

Isis has taken Iraq's second city Mosul and advanced south towards the capital in the past week, seizing Tikrit and parts of Baiji after national army and security forces fled. Isis leader Abu Bakr a-Baghdadi announced his attention to push on to Baghdad, but government forces have re-organised and begun pushing back.

Leading Shia cleric Ali al-Sistani urged volunteers to take up arms against the advancing militants during Friday prayers. His call was answered by thousands of young men across southern Iraq.

"The army and security forces around Baghdad are much more competent and organised than the guys that were up in Nineveh province, Tikrit and Salehuddin," said Jamie Ingram, an Iraq specialist at IHS. "There's also the presence of the Shia militias that are going to form the core components of this volunteer army."

While Isis's southward advance has slowed over the weekend, the group has opened a new battlefront in the north-west of the country.

Fierce fighting was reported in the town of Tal Afar, close to the Syrian border and home to a majority of Turkmen, on Sunday morning.

"The situation is disastrous in Tal Afar. There is crazy fighting and most families are trapped inside houses, they can't leave town," a local official told Reuters. "If the fighting continues, a mass killing among civilians could result."

US President Barack Obama said he was considering "options" in Iraq, while long-time adversary Iran has already sent thousands of troops and the chief of the Quds force to lead the fight against Isis.