A video uploaded onto social media on October 22 and which cannot be independently verified purports to show a Russian man, reading out a message in which he says he will be brutally killed unless a Saudi national is released.
In the video message the bearded man identifies himself as Sergei Gorbunov and sets a deadline of five days for the release of the detained Saudi national Khaled Suleiman in exchange for the Russian's freedom.
Suleiman is said to be a Saudi he said had been detained in the city of Hama.
"If I am not exchanged in five days, I will be butchered," he said in a trembling voice on a grainy video that was also aired on state-run Russian television on Thursday," he reads out in the video.
Russia has been Syrian President Bashar al-Assad's most powerful ally during a conflict which began with a crackdown on protests in 2011 but which has turned into a civil war in which more than 100,000 people have been killed.
An Arabic caption under the video, whose authenticity could not immediately be verified, said the man was captured by the Mujahideen of Ibn Taymiyyah Brigade.
It said he is asking Russia and Syria to agree a hostage swap between him and Khaled Mohammed Suleiman, who it says was captured in Hama by "the Alawite regime" - a reference to Assad's government.
"I appeal to presidents of Syria, Russia, the Red Cross ... I have not even started a family at home," the man said. I'm scared and I want to return to Russia, let someone help me."
The man said he was seized at an airport whose name was inaudible. He said the kidnappers, whom he identified only as "Muslims", had given him food and treated him well.
The caption accompanying the one-minute video said the Russian was an engineer who had been working in Syria. There was no immediate comment from the Russian government.
An official at the Russian embassy in Damascus, who asked not to be identified, told Reuters by telephone that diplomats were checking the information and refused further comment.