Brahimi is trying to negotiate an end to hostilities which have seem more than 40,000 people killed, as rebels battle to oust Assad.
Brahimi's visit comes after what activists said was a Syrian government air strike on Sunday, which resulted in dozens of deaths, they said.
UN envoy back in Syria to meet Assad (0:29)
Dec. 24 - UN envoy Lakhdar Brahimi leaves his hotel in Damascus to meet Syrian President Bashar al-Assad. Rough cut - no reporter narration. ( Transcript )