Brahimi, speaking in the Syrian capital of Damascus, said it would be impossible for the delayed talks in Geneva to occur without rebel participation, Voice of America reported.
"The Syrian government ensured its participation in the conference. The opposition, whether the coalition or other parties, are still seeking a way to be presented in the Geneva conference," Brahimi said.
Rebels demanded Syrian President Bashar Assad resign before they attend the conference, tentatively scheduled for later this month.
Brahimi said he hopes the issue can be resolved soon.
Syria's main rebel bloc, the National Coalition, will meet next week to determine its position. Some Islamist rebel factions warn they will consider any conference participants traitors.
Israeli warplanes attacked a military facility near the Syrian city of Latakia this week, an Obama administration official told CNN Thursday.
Targeted were missiles and related equipment the Israelis said they feared might be transferred to the militant group Hezbollah, the Washington official said.
Israel has been accused several times this year of conducting airstrikes inside Syria, including one in January in which Israeli fighter jets fired on a Syrian convoy suspected of moving weapons to Hezbollah, based in Lebanon.
The United Nations estimates more than 100,000 people have died since the Syrian conflict began in March 2011 after a government crackdown on peaceful protesters mushroomed into civil war.
Also Thursday, the Syrian military regained control of al-Safira, a town believed to house a chemical weapons production facility, CNN said. Rebels had withdrawn from the town, the Syrian Observatory for Human Rights said.
International inspectors are trying to ensure Syria eliminates its chemical weapons stockpile by mid-2014. Syria agreed to do so under international pressure.
Syria met a key benchmark of destroying all facilities where chemical weapons are produced and mixed, the Organization for the Prohibition of Chemical Weapons said Thursday.
Eight inspectors of the Joint OPCW-U.N. Mission inspected 21 of the 23 sites and 39 of the 41 facilities at those sites, OPCW said in a statement. The two remaining sites were not visited because of safety and security concerns. Syria declared those sites abandoned and said the chemical weapons material they held had been moved to other declared sites that were inspected, the OPCW said.