Baghdad, Iraq (BBN) – Iraq's most senior Shia cleric has issued a call to arms as Sunni-led insurgents seize more towns.

The call by a representative of Grand Ayatollah Ali al-Sistani came during Friday prayers, as the militants widened their grip in the north and east and threatened to march south, the BBC reported.

The UN says hundreds have been killed – with militants carrying out summary executions of civilians in Mosul.

The US and Iran have promised to help the fight against the insurgency.

Led by the Islamic State in Iraq and the Levant (ISIS), the insurgents have threatened to push to the capital, Baghdad and regions further south dominated by Iraq's Shia Muslim majority, including the holy cities of Najaf and Karbala, whom they regard as "infidels".

BBN/SSR/AD-13June14-6:29 pm (BST)