State-run television said women and children died in the attack in the Jaramana district of Damascus, the BBC reported.
Activists earlier reported government jets bombed Harasta, in the northeast area of the capital.
"The situation is bad and getting worse," he told reporters in Moscow after talks with Russian Foreign Minister Sergei Lavrov.
But it "will not discourage us because Syria is very important and the people of Syria deserve our support and interest," Brahimi said. "We think this civil war must end ... and the new Syria has to be built by all its sons."
More than 100 people, including 39 civilians, 34 rebel fighters and 35 security forces personnel, were killed Sunday, the Syrian Observatory for Human Rights, a British-based activist group, said.
The BBC said more than 420 people reportedly died since the cease-fire began Friday.
In Iraq, government officials said no weapons were found in an Iranian cargo flight bound for Syria, BNO News, FNA and others reported.
The Iraqi Civil Aviation Authority said it found only humanitarian aid, such as food, medical supplies and tents, and allowed the plane continue after ordering it to land at Baghdad International Airport for inspection.
The inspection was the second by Iraq's Shiite-led government this month of an Iranian cargo plane headed to Syria. The first one was Oct. 2. No weapons were found on that plane either.