Turkey is one of Assad's fiercest critics, a strong opposition backer and proponent of international intervention. It has been highly critical of Iran's stance during the Syrian uprising, which has killed around 60,000 people according to a U.N. estimate.
Turkey, Gulf Arab states, the United States and European allies support the mainly Sunni Muslim Syrian rebels, while Shi'ite Iran supports Assad, whose Alawite minority is an offshoot of Shi'ite Islam.
A pro-government newspaper said on Dec. 31 that Syrian forces arrested four Turkish fighter pilots who were trying to sneak into a military airport with an armed group in the northern province of Aleppo.
State-run media did not mention the reported incident.
TURKEY, QATAR INTERVENE
The al-Baraa brigade, part of the umbrella rebel organisation, the Free Syrian Army, said in October it would start killing the Iranians unless Assad freed Syrian opposition detainees and stopped shelling civilian areas.
But Qatar, following a request from Iran, urged the rebels not to carry out the threat.