The rocket shells struck the vicinity of a US 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 Islamic Resistance in Iraq from the Iraqi side of the Syrian-Iraqi border.
Also on Friday, the US 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 US 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 Islamic Resistance groups in Iraq and Syria have resumed their attacks on US 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 US bases in Syria, among which nine targeted the al-Shaddadi base.
AMK/PR