Kurdish militia attacked Islamist fighters southwest of their regional capital, Irbil, in the north of Iraq of Wednesday, backed, for the first time, by central government forces.

Thousands of Kurdish insurgents from neighboring Turkey and Syria attacked troops from the group formerly known as the Islamic State of Iraq and the Levant (ISIS) at Sinjar, a town on the road between Mosul and the Syrian border that fell to the Islamists over the weekend.

The fall of Mosul itself, Iraq’s second biggest city, two months ago shocked the government of Prime Minister Nouri al-Maliki and led to the Islamists conquering huge swathes of territory in an arc from Aleppo in Syria to near the western edge of Baghdad. The group subsequently rebranded itself the “Islamic State” while its leader, Abu Bakr al-Baghdadi, proclaimed himself to be the “caliph” — the historic title of the successors of the Prophet Muhammad who ruled the entire Muslim world.

In what appeared to be a coordinated offensive, peshmerga fighters local to Iraq’s autonomous Kurdistan Regional Government also pressed to lift the Islamic State’s siege of the Mosul Dam, the country’s largest single source of electricity.

The Kurdish counterattacks underline the increasing insignificance of the Middle East’s borders, especially those between Iraq and Syria.

One of the Islamic State’s objectives is to erase the frontier, drawn a century ago by European imperialists, and unite the Sunni Muslims living roughly between the Euphrates and Tigris Rivers in one country.

The Kurds, the world’s largest stateless nation, are scattered across Iraq, Syria and Turkey. While the Syrian regime, fighting its own civil war against mainly Sunni rebels, has largely left the Kurdish minority in the country to its own devices, allowing it to start building state institutions, Turkey, which has the region’s largest Kurdish population, is apprehensive about the prospect of a Kurdish state emerging — even as it backs autonomy for Iraq’s Kurds who are supplying it with oil.

Turkey’s Kurdistan Workers’ Party, known by its Kurdish acronym PKK, lent its support to the offensive in Iraq on Monday, saying in a statement, “All Kurds in the north, east, south and west must rise up against the attack on Kurds in Sinjar.”

In a remarkable turnaround, Iraq’s Maliki also backed the Kurds, promising air support. Although, given the limited capacity of the air force, it is unclear how much help this will be. The United States suspended the delivery of F-16 fighter jets to Iraq earlier this year while Sukhoi Su-25 aircraft, which are designed to provide close air support for ground forces, were only delivered by Russia in early July and may not be ready for combat yet.

The Iraqi leader, whose exclusion of the country’s Sunni Arabs from positions of power contributed to the sectarian tension that fueled the latest insurgency, had earlier accused the Kurds of supporting the Islamic State, then called ISIS. “We cannot be silent over this and we cannot be silent over Irbil being a headquarters for ISIS and Ba’ath and Al Qaeda and terrorist operations,” he said last month.

The Iraqi Kurds had just then taken advantage of the Islamist uprising to capture the city of Kirkuk and its surrounding oilfields.