Iraq stopped and searched a Syria-bound cargo plane from Iran for weapons for the second time in a month on Sunday, but allowed it to continue as no banned items were found, an official said.

The United States has been pressuring Baghdad to ensure all Iranian aircraft flying through its airspace are ordered to land and checked for weapons, but Iraq has said it will only stop planes when it has doubts about the cargo being transported.

"We ordered an Iranian cargo plane travelling to Syria to land at Baghdad International Airport for inspection," Nasser Bandar, the head of Iraq's civil aviation authority, told AFP.

"It was inspected by security forces, but we did not find any banned items, and we allowed it to continue its trip," Bandar said. Inspectors did not find any weapons, and instead found medical supplies and humanitarian goods.

"We have orders that any cargo plane we have doubts about, we must stop it and inspect it," Bandar said.

Iraq stopped an Iranian cargo plane for the same reason on October 2, but allowed it to continue to Syria after inspections concluded it was not carrying prohibited items.

On September 21, Iraq denied permission for a North Korean aircraft to transit its airspace on its way to Syria over suspicions it would carry arms and advisers there.

Iraq has pointedly avoided calling for the departure from office of Syrian President Bashar al-Assad, who is locked in a bloody civil war with rebels opposed to his regime, and has instead urged an end to violence by all parties.