Beirut-based producer Ayat Basma and cameraman Ezzat Baltaji had been expected to cross into Lebanon by road at approximately 1830 GMT (2:30 p.m. ET) on Saturday, where they had arranged for a taxi to pick them up from the border.
The last known contact was at 1722 GMT (1:22 p.m. ET), when Baltaji sent a phone message to a colleague in Beirut in which he said: "We will leave now."
Basma and Baltaji, both Lebanese nationals, travelled to Syria on Thursday afternoon. Mass protests that erupted 10 days ago have posed the biggest challenge to President Bashar al-Assad's 11-year rule.
The two journalists have been unreachable by telephone since Saturday night.
On Friday, Syrian authorities withdrew the accreditation of Reuters text correspondent Khaled Yacoub Oweis, saying he had filed "unprofessional and false" coverage of events in Syria.