"These measures are imposed on Syrian trucks only, while Iraqi and Turkish trucks are passing easily," said the Arab Organisation of Human Rights in Syria (AHORS) in a statement.
It said the Syrian trucks had been stranded near the Yarubia border post for three days in difficult circumstances due to a lack of water and extreme temperatures and charged that their flow was subject to "the mood of these forces".
The group's spokesman Ammar Qurabi said the flow of trucks from Syria to Iraq had not been hampered.
Ties between Syria and Iraq have been strained because of repeated accusations by Iraqi and US officials that Damascus was not doing enough to prevent the flow of fighters into Iraq to fight US-led forces.
Some drivers say they are being
used as human shields
Syria, which denies the charges and says it wants solid ties
with Baghdad, shares a long desert border with Iraq straddling about 600km.
"These extraordinary measures by American forces at the Syrian-Iraqi border represented a violation of the rights of Syrian citizens in Iraqi territory," it said.
AHORS said some trucks were allowed to pass after their drivers paid off Iraqi guards.
It quoted some drivers as saying they suspected they were being used as human shields for a nearby US military base.
"Some drivers spoke of the possibility that they are being used as shields in view of their extreme vicinity to an American military base, about 40m away," it said.
The flow of trucks from Lebanon to Syria has also been hampered in recent weeks. Damascus said it had introduced security measures after explosives were found on a truck bound for its territory.
Lebanese politicians have said that tight Syrian border controls were the result of tensions between Beirut and Damascus after Syria pulled out its troops from Lebanon in April.
Syria allowed trucks stranded for weeks to pass after a visit by new Lebanese Prime Minister Fuad Siniora to Damascus last month.