Three crates from an arms manufacturer - addressed to Saudi Arabia - have been seen in a base being used by rebel fighters in the city of Aleppo.
How the small crates reached Aleppo is unknown, and the BBC was not allowed to film their contents.
Saudi Arabia has refused to comment on the matter.
The crates of ammunition found in an Aleppo mosque were made by the Ukrainian firm Dastan, which specialises in naval weapons and missile complexes. The LCW (the factory manufacturer) main product is 7.62 mm bullets for AK-47 which are widely used by both - Syrian army and the rebels.
What was in the crates is unknown, says the BBC's Ian Pannell, who has been in Aleppo, as is how they ended up there.
But their presence clearly suggests that someone in the Gulf is actively helping the rebels fighting to overthrow President Bashar al-Assad, our correspondent says.
When contacted, Saudi officials refused to comment.
The BBC's security correspondent Frank Gardner says Saudi Arabia generally prefers to conduct its foreign affairs through low-key, behind-the-scenes discretion.
The apparent discovery of Saudi ammunition in a Syrian mosque could attract unwelcome attention, he adds.
Privately, opposition sources have confirmed to that they are receiving assistance from Saudi Arabia and Qatar.
The New York Times reports that Saudi and Qatari officials are sending small arms to the rebels, but are holding off sending heavier equipment, such as shoulder-fired missiles.
This is in part because they have been discouraged by the United States, which fears the heavier weapons could end up in the hands of terrorists, the newspaper says.