US President Barack Obama has said he will take several days to decide what action to take over Iraq, but that no US troops will be deployed there.

Any US involvement “has to be joined by a serious and sincere effort by Iraq’s leaders to set aside sectarian differences”, he said.

In recent days Sunni insurgents have seized the cities of Mosul and Tikrit, and are moving closer to Baghdad.

Iraq’s most senior Shia cleric has issued a call to arms to fellow Shias.

The message from Grand Ayatollah Ali al-Sistani, which was read out at Friday prayers in Karbala, said: “Citizens who are able to bear arms and fight terrorists, defending their country and their people and their holy places, should volunteer and join the security forces to achieve this holy purpose.”