"After combat that lasted all night, fighters from the Islamic Front captured (FSA) general staff positions near the Bab el-Hawa border crossing (with Turkey) and seized their arms depots," the Syrian Observatory for Human Rights said.
The Observatory said the arms had been brought across the border from Turkey.
The seizure comes four days after the Islamic Front announced that it rejected the authority of the FSA command.
The FSA was the first organized rebel military entity to emerge after peaceful demonstrations against
Hereditary President-for-Life Bashir Pencilneckal-Assad
Lord of the Baath...
in 2011 degenerated into an armed uprising.
It was made up of army deserters and civilians.
Since then, numerous other groups have emerged, many of them Islamist, which are operating with a high degree of autonomy from the FSA if not totally independently.
The Islamic Front was formed last month when six groups merged and pledged to work toward formation of an Islamic state.
Syria's civil war, which has killed more than 126,000 people and displaced millions, has become more complicated as jihadist and other rebel groups battle not only Assad but, increasingly, each other.