U.S.-backed Syrian Kurdish forces began their assault on an Islamic State-held air base in northern Syria on Sunday, activists said, as reports circulated that residents were evacuating the nearby city of Raqqa, the militants' de facto capital.

The Kurdish-led Syrian Democratic Forces have ratcheted up the pressure on IS in Syria since the U.S. airlifted hundreds of SDF fighters, U.S. military advisers, and U.S. artillery behind IS lines on March 21.

SDF fighters reached the limits of the Tabqa air base Sunday and began firing on militants inside, the Britain-based Syrian Observatory for Human Rights monitoring group reported, while the activist-run Raqqa is Being Slaughtered Silently said the SDF now "controlled" several parts of the air base.

The two groups rely on local contacts to smuggle information out of IS-held territory.

The media arm for the Kurdish PYD political party, which is aligned with the SDF, says fighters have seized a village 0.6 miles away from the air base.

The air base lies 28 miles west of Raqqa. Other SDF forces are within 6 miles of the city, approaching from the north.

Meanwhile there were conflicting reports over whether civilians had begun evacuating Raqqa due to concerns over the stability of the nearby Tabqa Dam.

The militants said U.S.-led coalition airstrikes had locked up the dam's gates, causing the water level behind it to rise. Raqqa is Being Slaughtered Silently reported that IS had ordered Raqqa residents to evacuate, though without their furniture. Tabqa Dam is 25 miles upstream of Raqqa on the Euphrates River.

The coalition could not immediately be reached for comment.

But the Observatory said there were no evacuations happening, as did the activist-run Raqqa 24 media center.

Raqqa 24 said engineers employed by the militants had restored power to the dam's gates and said the structure was functioning normally.

The reports from Raqqa came as a leading Syrian opposition group called on the U.S.-led coalition to stop targeting residential areas in and around the city.

The Syrian National Coalition said in a statement that it was "increasingly concerned" about civilian casualties in the campaign against the extremist group. The exiled opposition coalition is taking part in U.N.-mediated talks in Geneva.

The SNC said it believed coalition forces were behind an airstrike that killed at least 30 civilians sheltering in a school in the countryside outside Raqqa on March 21. The coalition has said it is investigating.

The U.S. has provided substantial air and ground support to the Kurdish-led Syrian Democratic Forces, who are closing in on Raqqa as well as the Tabqa Dam.

The Britain-based Syrian Observatory for Human Rights said coalition airstrikes have killed 89 civilians in Raqqa province in the past week, including 35 in a school in the village of Mansoura.