BEIRUT — Islamic State fighters captured a major military air base in northeastern Syria on Sunday, eliminating the last government-held outpost in a province dominated by the extremist group, activists and state media said.

The jihadis launched their long-anticipated offensive last week to seize the sprawling Tabqa air field, located some 25 miles from the extremists’ stronghold in the city of Raqqa. The air base was one of the most significant government military facilities in the area, containing several warplane squadrons, helicopters, tanks, artillery and ammunition.

After several failed efforts to breach the facility’s walls in recent days, Islamic State fighters managed to punch through and storm the air field Sunday, the Britain-based Syrian Observatory for Human Rights said. Despite government airstrikes to try to beat back the attack, the jihadis quickly quashed the remaining pockets of resistance, killing dozens of soldiers.

“Some of the Syrian regime troops pulled out, and now the Islamic State is in full control of Tabqa,” said Observatory director Rami Abdurrahman. “This makes Raqqa province the first to fully fall out of government hands.”

Another activist group, the Local Coordination Committees, also said the extremist group was in control.

The SANA state news agency confirmed that the government had lost the air base, saying troops “are successfully reassembling after evacuating the airport.”

Islamic State fighters had been pressing in on the base for weeks. When the fight finally came, it was bloody.

Since seizing control of much of northern and western Iraq in June and declaring the establishment of self-styled caliphate, the Islamic State group has also succeeded in consolidating its hold on a huge tract of northeastern Syria.

It has overwhelmed outposts held by rival rebels in Deir el-Zour province, which borders Iraq, while also systematically picking off isolated government bases in the northeast, decapitating army commanders and pro-government militiamen and putting their heads on display.

Last month, Islamic State fighters overran the sprawling Division 17 military base in Raqqa, killing at least 85 soldiers. Two weeks later, the extremists seized the nearby Brigade 93 base after days of heavy fighting.