Spokeswoman Jennifer Psaki said Washington was "obviously concerned" about the Friday move by the forces of the Islamic Front, a new coalition of six major Islamist rebel groups, during which they took over the headquarters and warehouses in Idlib province at the Bab al-Hawa border crossing with Turkey, Xinhua reported.
The US was working with the SMC to "inventory the status of US equipment, of supplies provided to the SMC," said Psaki at a regular news briefing.
"As a result of this situation, the United States has suspended all deliveries of non-lethal assistance into northern Syria, while we evaluate the situation on the ground and gather additional details."
According to the spokeswoman, meals ready to eat and laptops could be among items in the lost warehouses.
The US has committed $250 million in non-lethal aid to the Syrian National Coalition, the main opposition umbrella group in exile, as well as the SMC and other local groups.
Britain also announced its decision to hold off non-lethal aid to northern Syria on Wednesday.