DAMASCUS, Dec. 1 (Xinhua) -- Rocket shells resumed targeting U.S. military base in northeastern Syria on Friday as part of the "revenge campaign for Gaza," a war monitor reported.

The rocket shells struck the vicinity of a U.S. base near al-Shaddadi city in northeastern Syria on Friday, according to the Syrian Observatory for Human Rights.

The observatory said that the shells were believed to have been launched by the Iran-backed Islamic Resistance in Iraq from the Iraqi side of the Syrian-Iraqi border.

Also on Friday, the U.S. forces sent backup troops to another military base in the Kharab al-Jir town, and a significant influx of military reinforcements was delivered to the base by U.S. cargo planes, the observatory added.

The Kharab al-Jir base is used as a station for gathering vehicles entering and exiting Syrian territories, due to its proximity to the Syrian-Iraqi border.

The UK-based watchdog group said the Iranian-backed groups in Iraq and Syria have resumed their attacks on U.S. bases within Syrian territory, as part of the "revenge campaign for Gaza."

Since October 19, the war monitor said it had documented 46 attacks on U.S. bases in Syria, among which nine targeted the al-Shaddadi base.

The U.S. forces have established bases in several areas in northeastern Syria, especially in oil and gas fields. The Syrian government repeatedly charged that the U.S. is aiming to steal and smuggle oil and gas production abroad using convoys of tanks and trucks, in coordination with the Kurdish-led Syrian Democratic Forces.