ISIS forces captured the Tabqa airbase in northern Syria on Sunday, the last remaining military defense in the region, bolstering the Sunni militants' control on the Raqqa Province. The province's capital, Raqqa, has long served as the group's headquarters, the New York Times reports; according to the BBC, the city "is believed to hold dozens of warplanes, helicopters, tanks and artillery."

From the New York Times:

The Syrian Observatory for Human Rights, which tracks the conflict from Britain through a network of contacts inside Syria, said ISIS' attack on Sunday was its fourth in the past week. The Syrian government had launched airstrikes on ISIS positions, but the group's fighters managed to enter the grounds of the air base on Sunday and took it over after many of the troops inside withdrew.

More than 340 ISIS fighters have been killed since the start of the offensive on Tuesday, the Observatory said, in addition to about 170 government soldiers. If confirmed, those numbers would make the battle the deadliest yet between the jihadist movement and the Syrian government.

Witnesses at the base told the Guardian that many of the Syrian soldiers captured by the militants had been beheaded, their severed heads displayed in victory and as a grisly warning. Analysts told the BBC that the airbase's capture is "a significant blow" to Syrian President Bashar al-Assad and his government's ability to defend itself in the country's northern territories.

[Image via AP]