In the biggest blow to Iraq’s central government since the fall of Mosul a year ago, Islamic State militants on Sunday said they had taken control of Ramadi, the country’s tenth largest city and the capital of the mostly Sunni Anbar Province.

The same day, Prime Minister Haider al-Abadi signed off on the deployment of Shia volunteer militias to take back the city, something he previously resisted for fear of provoking a sectarian backlash.

The self-proclaimed Islamic State feeds on Iraqi Sunnis’ dissatisfaction with the way they are governed. Abadi’s predecessor, Nouri al-Maliki, was blatantly pro-Shia and purged Sunni officers and officials from the army and state during his eight years in power. The incumbent premier, also a Shia, has promised to do better but the central government in Baghdad is still dominated by members of his own sect.

Abadi’s administration had vowed to liberate Anbar after routing the militants from the city of Tikrit last month. But the news agency Reuters reports that the security forces have struggled to gain traction in the vast desert province which borders Jordan, Saudi Arabia and Syria.

The United States, which support Abadi’s efforts against the Islamic State, downplayed the fall of Ramadi. A Defense Department spokeswoman said the loss of the city would not mean the overall military campaign was turning in the Islamic State’s favor. “That just means the coalition will have to support Iraqi forces to take it back later,” she said.

The seizure of Ramadi would nevertheless be the Al Qaeda offshoot’s biggest victory since it conquered Mosul, Iraq’s second city, in June last year. The fall of Mosul triggered a bloody sectarian conflict that now spans the eastern half of Syria as well as the northwest of Iraq.

The Islamic State has proclaimed a caliphate in the areas it controls and battles both the Iraqi government and the secular regime of President Bashar al-Assad in Damascus. One of its objectives is to erase the border between Iraq and Syria, drawn a century ago by European imperialists, and unite the Sunni Muslims living roughly between the Euphrates and Tigris Rivers in one country.

Arab and Western allies launched airstrikes against the group in August. European countries have also supplied the semi-independent Kurdistan Regional Government in the north of Iraq with combat equipment.

However, Kurdish commanders complain they remain ill-equipped against the militants who plundered Iraqi army depots when they overran Mosul.