The Lebanese officials, who spoke on condition of anonymity because they were not authorized to brief reporters, said Brahimi was expected to meet Syria's foreign minister later Sunday and President Bashar Assad on Monday.
The trip is Brahimi's third since taking the job following the resignation of former U.N. Secretary General Kofi Annan after both sides disregarded a cease-fire he brokered in April.
While not advancing a comprehensive peace plan, Brahimi has called on the sides to negotiate a solution.
The security situation has gotten notably worse for the regime since his last visit, with rebels storming a number of military bases and seizing valuable munitions. Russia, Assad's most powerful international backer, also appears to have changed his assessment of Assad's strength, as top officials say they do not seek to preserve his regime, while still calling for a negotiated solution.
Still, neither side appears willing to talk.
In a lengthy Sunday news conference, Syrian Information Minister Omran al-Zoubi repeated the Syrian government's line that it is fighting terrorist groups backed by foreign powers who seek to destroy Syria.
Al-Zoubi said the government was willing to engage in dialogue but said the other side wasn't.
"We speak of dialogue with those who believe in national dialogue," he said. "But those who rejected dialogue in their statements and called for arms and use of weapons, that's a different issue. They don't want dialogue."
Rebel groups refuse to talk to Assad, saying too many people have died for him to be considered part of the solution.
Violence raged elsewhere in the country on Sunday. Anti-regime activists reported government airstrikes on suburbs east of the capital and the northern province of Aleppo.
Airstrikes on the town of al-Safira, south of Aleppo, killed 13 people, including a mother and five daughters from one family, a local activist named Hussein said via Skype. He gave only his first name for fear of retribution.
The town lies next to a large military complex with factories and artillery and air defense bases. Hussein guessed the airstrike was payback for recent rebel attacks on the complex.
"The strikes don't hit the fighters at all," he said. "They want to take revenge on the civilians."
The Observatory said at least 10 rebels and an unknown number of government troops were killed in clashes in Afreen, near Aleppo, Syria's largest city, as rebels sought to storm an army base there.
Anti-regime activists say more than 40,000 people have been killed since Syria's crisis began in March 2011.
Associated Press writer Albert Aji contributed reporting from Damascus.