UN-Arab League envoy Lakhdar Brahimi flew in to Beirut on Monday on his way to Damascus to try to whip up support for a peace conference on Syria, airport sources said.
Brahimi arrived from Tehran, where he said it was "necessary" for Iran, a key ally of the Damascus regime, to take part in an international peace conference aimed at ending Syria's two-and-a-half-year conflict.
Brahimi was to travel overland to Damascus, making his first visit to Syria since last December.
At that time, Brahimi called for "real change" in Syria, and for all powers to be handed over to a transitional government.
Official media in Syria responded to his remarks by branding him as biased and mocking him as an "ageing tourist".
This time around, Damascus has said the 79-year-old is welcome so long as he stays "impartial".
Brahimi was appointed the UN and Arab League envoy to Syria in September 2012, after Kofi Annan quit the post.
His visit on Monday comes after 19 powerful rebel groups warned that any opposition blocs which agreed to negotiate with Syrian President Bashar al-Assad's regime would be regarded as traitors.