Brahimi commented on the pace of the talks following the first-to-face meeting of the two sides, the New York Times reported.
He had met separately with each delegation Monday as the second full week of talks got underway.
Brahimi decided to bring the groups together Tuesday, saying the meetings had produced some "common ground," CNN reported.
Afterward, Brahimi told reporters "I don't have much to tell you except that the beginning of this week is as laborious as it was the last week. We are not making much progress."
Both sides did agree to extend the three-day truce in the Old City section of the besieged city of Homs to allow humanitarian access after a cease-fire was broken Saturday, Turkey's Cihan News Agency reported.
More than 1,100 residents have been evacuated from the city, the United Nations said in a statement Tuesday.
More than 300 male evacuees between the ages of 15 and 55 are being questioned by authorities in an abandoned school. More than 40 have been released, the United Nations said, and have been taken to shelters in Al Waer, a Homs suburb.
The Syrian Arab Red Crescent, which is helping to coordinate the response, said shelters were set up in several areas to receive people wishing to leave Homs, which has gotten little food or aid for nearly two years.
Since Friday, humanitarian teams, working with community leaders, local authorities, and representatives of all sides involved in the civil war, evacuated more than 800 people and brought food and medical supplies to those who decided to remain in the city.
Notable deaths of 2014 [PHOTOS]