Islamic State militants Sunday captured the Tabqa air base, the Syrian military's last standing army base in northern Raqqa, human rights activists said. Rami Abdul-Rahman, the head of the Observatory for Human Rights in London, said members of militant group, formerly known as the Islamic State in Iraq and Syria, or ISIS, took full control of the facility after a weeklong attempt to seize the base.

Some Syrian government forces have withdrawn from the base. Syrian state media said that government forces were "regrouping" from heavy fighting with ISIS members after evacuating the airport.

The extremist group had reportedly made at least three previous attempts to storm Tabqa since Tuesday, resulting in the deaths of 100 ISIS fighters and 25 Syrian army soldiers, the observatory said. Raqqa, with a population of about 1 million people, is ISIS's main power base in Syria. The Islamic State has declared an Islamic caliphate in the parts of Iraq and Syria under its control.