Iran has sent 2,000 advance troops to Iraq in the past 48 hours to help tackle a jihadist insurgency, a senior Iraqi official has told the Guardian.

The confirmation comes as the Iranian president, Hassan Rouhani, said Iran was ready to support Iraq from the mortal threat fast spreading through the country, while the Iraqi prime minister, Nouri al-Maliki, called on citizens to take up arms in their country's defence.

Addressing the country on Saturday, Maliki said rebels from the Islamic State of Iraq and the Levant (Isis) had given "an incentive to the army and to Iraqis to act bravely". His call to arms came after reports surfaced that hundreds of young men were flocking to volunteer centres across Baghdad to join the fight against Isis.

In Iran, Rouhani raised the prospect of Teheran cooperating with its old enemy Washington to defeat the Sunni insurgent group – which is attempting to ignite a sectarian war beyond Iraq's borders.