Syrian opposition fighters are holding the bodies of 12 Iranian Revolutionary Guard advisers killed in Syria last week, an Iranian military official said on Tuesday according to the ISNA news agency.

Thirteen Iranians were reported killed and 21 others wounded last week in heavy fighting in Khan Tuman south of the the battleground city of Aleppo.

It was Iran's biggest loss of forces within a very short period, based on official figures. All of the advisers were reportedly from Iran's northern province of Mazandaran.

All other members of the forces from Mazandaran, including nine who were injured, have now returned to Iran, said Hossein Ali Rezayi, a Guards spokesman in the region.

But given the ongoing fighting in northern Syria, Rezayi said, the bodies of the remaining guards would only be able to be repatriated "after the liberation of those areas".

Forces loyal to Syrian President Bashar al-Assad including Hezbollah fighters drove rebels out of Khan Tuman in December, but on Friday members of the al-Qaeda-affiliated Nusra Front and their allies retook the area, a key village which could open up new supply routes for rebel-held areas of Aleppo. Dozens were reported killed on both sides.

Five or six members of the Iranian forces were captured in the battle, Iranian conservative politician Esmail Kossari told the judiciary's official news service Mizan Online on Tuesday.

No senior military official or politician has confirmed the capture.

Iran's Fars news agency on Tuesday reported the deaths of four other Iranians, including a general, Shafi Shafiei.

The commander was killed on Friday in Khan Tuman, Mizan said.

Three Afghan volunteers killed in Syria were buried Monday in the northeast Iranian city of Mashhad, papers reported.