The Syrian government will free 2,130 civilian prisoners on Wednesday in exchange for the release of 48 Iranians held by Syrian rebels, a board member of the Turkish humanitarian aid agency IHH said.
The Syrian rebel al-Baraa brigade seized the Iranians in early August and initially threatened to kill them.
"Most of the prisoners to be swapped are Syrian citizens in exchange for Iranians, and there are a few Turks as well," IHH board member Osman Atalay told Reuters.
Citizens of other countries were also among those to be released by the Syrian government, he said.
He quoted IHH chief Bulent Yildirim, who was in Damascus to help coordinate the deal, as saying that the prisoner swap had already begun.
There was no confirmation from the Syrian government.
Turkey's state-run Anatolian news agency said the deal was brokered by Turkey and Qatar.
The rebels say the captives were Revolutionary Guards sent by Iran to help Syrian President Bashar al-Assad's forces crush their revolt. Iran, one of Assad's closest allies, has said they were pilgrims visiting Shi'ite Muslim shrines.
The IHH came to prominence in May 2010 when Israeli marines stormed its Mavi Marmara aid ship to enforce a naval blockade of the Palestinian-run Gaza Strip and killed nine Turks in clashes with activists on board.
The IHH has also played a role in the previous release of Turkish and Syrian citizens in Syria.