BEIRUT — Syrian government forces are besieging the last Islamic State stronghold in the northern province of Aleppo, weeks after launching an offensive to retake the entire province, state media and an opposition monitoring group reported Thursday.

The push on Deir Hafer comes as U.S. aircraft ferried Syrian Kurdish fighters and allies behind Islamic State lines to spearhead a major ground assault on the strategic town of Tabqa in Raqqa province, which borders Aleppo.

That airlift marked a deepening U.S. involvement in Syria’s conflict ahead of a looming battle for the extremist group’s de facto capital, the city of Raqqa.

The airlift was part of what Pentagon spokesman Eric Pahon described as a large, high-priority offensive to secure the area around Tabqa and the associated Tabqa Dam.

The twin offensives follow weeks of mounting pressure on the opposition to surrender in pockets outside Damascus. That campaign has been underway despite a nominal cease-fire signed last December.

A spokesman for the powerful Ahrar al-Sham rebel faction described the twin offensives as a message to negotiators in Geneva that “the revolutionaries are able to overturn the scales and return to a military solution.”