The fighting erupted at Thursday dawn after an assault by the terrorist Islamic State of Iraq and the Levant (ISIL) on posts held by Al-Qaeda affiliate Al-Nusra Front and its allies.
The clashes centered around the border town of Albu Kamal, from which ISIL was expelled from earlier this year, said the so-called Syrian Observatory for Human Rights.
It said 51 combatants were killed.
The bodies of another 10 militants, apparently executed by ISIL, were found at two sites near Albu Kamal, the Observatory said, adding there were reports of other summary executions in the same area.
Earlier, the monitoring group's head Rami Abdel Rahman told AFP that ISIL was advancing in Albu Kamal and had taken control of several districts previously held by Al-Nusra and other terrorist brigades.
The clashes prompted soldiers on the Iraqi side of the border to reinforce their positions.
The border crossing itself on the Syrian side remained in the hands of the so-called Free Syrian Army, according to a rebel chief and an AFP reporter on the Iraqi side.
BA/BA