A militant rocket attack in eastern Syria on Friday targeted a bus with oil industry employees, killing at least 10, the government said. To the north, Syrian Kurdish-led forces announced they arrested 52 militants in an operation against the Islamic State group’s sleeper cells.

According to Syria’s petroleum ministry, the rocket struck in the Al-Taym gas field in eastern Deir el-Zour province. There was no immediate claim of responsibility but the Britain-based Syrian Observatory for Human Rights, an opposition war monitoring group, said IS was behind the attack.

The Observatory also reported a higher death toll from the rocket attack, saying at last 12 workers were killed.

Also Friday, the U.S.-backed and Kurdish-led Syrian Democratic Forces said their raids had reportedly thwarted an attack planned for New Year’s Eve. The IS militants were hiding in residential areas and farms, a statement from the forces said.

The yearslong U.S.-backed campaign had succeeded in crushing the militants’ territorial control in Iraq and Syria, but IS fighters maintain sleeper cells and have staged attacks that have killed scores of Iraqis and Syrians in the past months.

On Thursday, the Syrian Kurdish-led forces announced their operation, citing a surge in IS attacks and saying that “Operation Al-Jazeera Thunderbolt” aims to target sleeper cells in al-Hol and nearby in Tal Hamis areas.

Since 2011, Syria has been mired in a bloody civil war that has drawn in regional and global powers. Syrian President Bashar Assad has mostly regained control of the country, but parts of its north remain under the control of rebels, as well as Turkish and Syrian Kurdish forces.

Also, some 900 U.S. troops in Syria support the Kurdish-led forces' fight against IS and have frequently targeted IS militants, mostly in parts of northeastern Syria under Kurdish control.

The U.S. Central Command on Thursday reported conducting some 313 operations against IS in 2022 in Syria and Iraq, mostly in cooperation with Kurdish-led forces. According to a CENTCOM statement, 215 militants from the Islamic State group were arrested and 466 were killed in Syria.