Rami Abdulrahman, who runs the Syrian Observatory for Human Rights, told Reuters the raids had hit the border town of Albu Kamal and surrounding areas.
It was not clear who had mounted the air raid. A U.S.-led alliance launched air strikes on Islamic State in Syria on Tuesday. A spokesman for the U.S. military said those strikes were "only the beginning".
"The people there, the activists, say they (the strikes) are probably the (international) coalition, not the regime," Abdulrahman said, referring to the Syrian government. "The strength of the explosions are greater. Like yesterday."
The area around Albu Kamal in eastern Syria has been the focus of heavy bombing by U.S-led forces. The Observatory said around 22 strikes hit the area on Tuesday.
Albu Kamal is one of the main border crossings between Iraq and Syria along a frontier that Islamic State wants to erase after seizing territory both sides.
Weaponry seized by Islamic State in Iraq during a lighting offence in June and taken across the border has helped the group secure more land in Syria.(*)