Iraqi PM Nouri Maliki has fired senior officers for failing to halt a sweeping advance by Sunni Islamist rebels. Four army commanders were dismissed because they did not perform "their national duty", a government statement said on Tuesday. Iraqi forces have been engaged in heavy clashes with the rebels who have seized several key cities in the past week. The US is deploying up to 275 military personnel to protect staff at its huge embassy in the capital, Baghdad. Mr Maliki and other senior figures of his Shia government were joined by Sunni leaders in a call for "national unity", after talks in Baghdad on Tuesday evening. They urged Iraqis to avoid sectarian grievances and said individuals with no official state function were banned from carrying weapons. The militants, led by the Islamic State in Iraq and the Levant (ISIS), took control of the northern cities of Mosul and Tikrit in a rapid advance last week, and Tal Afar on Monday. They also briefly captured parts of the city of Baquba - just 60km (35 miles) from Baghdad - in an overnight assault, before government troops and allied Shia militia pushed them back on Tuesday. Qasem Suleimani, the commander of an elite unit of Iran's revolutionary guards, is reported to be in Baghdad, helping military and Shia leaders co-ordinate their campaign against the rebels. 'Defeat this enemy' The ISIS advance has also raised international concern over regional stability. The United Nations has warned of "a real risk of further sectarian violence on a massive scale in Iraq and beyond its borders". ISIS fighters have been accused of carrying out hundreds of summary executions since their offensive began last week, and Sunni militants have posted photos online appearing to show fighters carrying out massacres of captured Iraqi soldiers. US Vice-President Joe Biden said on Tuesday "urgent assistance [was] clearly required", but stressed that Iraqis had to "pull together to defeat this enemy". Although Washington has already ruled out sending in ground troops to fight alongside Iraqi troops, drone strikes remain a possibility. The aircraft carrier USS George HW Bush has been deployed to the Gulf, along with two other US warships. Earlier, US officials also held brief discussions about Iraq with their Iranian counterparts at a meeting in Vienna about Tehran's nuclear programme. However, the US authorities were quick to dismiss reports of military collaboration with a major foe.                                                                                                                                                                               BBC