BEIRUT (Reuters) - U.S.-backed Syrian militias accused the Syrian government on Monday of bombing their positions southwest of Raqqa city in recent days and threatened to retaliate if the attacks continue.

A military vehicle of SDF in west of Raqqa province,Syria June 18, 2017.REUTERS/ Rodi Said

“The regime’s forces have mounted large-scale attacks using planes, artillery, and tanks since June 17,” Syrian Democratic Forces spokesman Talal Selo said in a statement.

Selo said the attacks targeted SDF areas near the town of Tabqa and the adjacent dam, Syria’s largest, which the alliance of Kurdish and Arab militias captured from Islamic State militants in May.

There was no immediate comment from the Syrian army.

The Kurdish-led SDF began this month to push into Raqqa, Islamic State’s base of operations in Syria. With air strikes and special forces from the U.S.-led coalition, they had been encircling the city since November.

“If the regime continues attacking our positions in Raqqa province, we will be forced to retaliate...and defend our forces,” Selo said, accusing the government and its allies of trying to obstruct the Raqqa offensive.

The Syrian government has previously said it considers Deir al-Zor and the Badia region, south of Raqqa, as its military priorities, signaling that it did not intend to challenge the SDF’s Raqqa campaign.

On Sunday, a U.S. warplane shot down a Syrian army jet southwest of Raqqa, with Washington saying the army had dropped bombs near the SDF. Damascus said the plane was downed while flying a mission against Islamic State.

The Syrian army said the plane crashed and the pilot was missing in the first such downing of a Syrian jet by the United States since the start of the Syrian conflict in 2011.

The U.S. Central Command said the Syrian plane was downed in self-defense after “pro-Syrian regime forces” attacked an SDF-held town near Tabqa.

Islamic State has been on the back foot in Syria over the past year, losing territory to separate military campaigns, including by the SDF and by the Russian-backed Syrian army.