Iraq says it has intensified a counter-offensive against Sunni rebels who have taken control of large parts of the country.

The rebels, lead by the Islamic State of Iraq and the Levant, have seized one entire province and parts of three others, since they launched their offensive last week.

The US has responded to the crisis by sending a carrier strike group to the Gulf, led by the George WH Bush aircraft carrier, as Iran warned against any intervention in Iraq.

Iraqi security forces had collapsed in face of the initial onslaught, with many abandoning their vehicles and weapons caches to the rebels

Iraqi commanders said the army had started to push back Sunni rebels and that soldiers had recaptured two towns north of Baghdad.

Al Jazeera's Imran Khan, reporting from Baghdad, said there was heavy fighting between rebels and security forces in Tal Afar, a mainly Turkmen Shia town 60km west of Mosul, and in Diyala province, which borders Iran.

There were no confirmed figures on casualties, but Shia tribal militias had suffered heavy casualties in Diyala, Khan said.

Volunteers targeted

Attempts to subdue the uprising will be bolstered by a flood of civilian volunteers, encouraged by a call to arms by the country's leading Shia cleric, Grand Ayatollah Ali al-Sistani.

The government echoed the call, asking all military-aged males to sign up to fight the rebels.

A centre for new recruits in Khales, in central Iraq, was hit by mortar fire on Sunday, killing six people including three soldiers, the AFP news agency reported.

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Iran has warned that any "foreign military involvement in Iraq" would only complicate the crisis.

"Iraq has the capacity and necessary preparations for the fight against terrorism and extremism," Foreign Ministry spokesman Marzieh Afkham said, according to the ISNA news agency.

"The people and government of Iraq will be able to neutralise this conspiracy," Afkham said.

The Foreign Ministry's comments come a day after the president, Hassan Rouhani, said that Iran had not been asked for help by its neighbour.

The rapid gains the rebels achieved have brought renewed focus on the conflict in Syria and its role in precipitating the crisis in Iraq.

The former UN mediator in Syria, Lakhdar Brahimi, said the international community's inability to resolve the war in Syria had caused the conflict to spread to Iraq.

"A conflict of this kind cannot stay confined within the borders of one country," said Brahimi, who resigned from as UN-Arab League representative to Syria in May.

ISIL is also involved in the conflict in Syria, fighting both the government and opposition factions including al-Qaeda's affiliate in the country, Jabhat al-Nusra.