Lakhdar Brahimi's contract was due to expire Friday. It has been renewed through the rest of 2013, U.N. spokesman Martin Nesirky said Thursday.
All international efforts to end Syria's civil war have failed amid divisions within the U.N. Security Council on how to approach the conflict. Brahimi, a 79-year-old Algerian diplomat, replaced Kofi Annan as envoy in August after the former U.N. secretary-general failed to broker a cease-fire.
The war claimed at least 53 more lives Thursday when a massive car bomb exploded near Syria's ruling party headquarters in Damascus. While no group has claimed responsibility, the attack suggested that rebel fighters are resorting to guerrilla tactics to loosen President Bashar Assad's grip on the capital.
Brahimi, who is known as a strong-willed and independent broker, said Sunday that a recent opposition offer to negotiate "challenges the Syrian government to fulfill its often-repeated assertion that it is ready for dialogue and a peaceful settlement."
The opposition Syrian National Coalition said Friday that it would not allow Assad or members of his security services to participate in talks.
"This initiative is on the table and will be on the table," Brahimi said in Cairo following talks with Arab League chief Nabil Elaraby. "We believe that if a dialogue begins in one of the U.N. headquarters, at least initially, between the opposition and an acceptable delegation from the Syrian government, it will be a start for getting out of the dark tunnel in which Syria is placed."
Syria has accused Brahimi of "flagrant bias" after he called for real change and said Assad was resisting his people's aspirations.
A U.N. report released last week said Syria's civil war is becoming increasingly sectarian and both sides are becoming more radicalized. The report urged the international community to curb the supply of weapons to anti-government forces.
Several European Union foreign ministers echoed that view at a meeting in Brussels this week, saying they were opposed to sending any more arms into Syria. The foreign ministers said they held out hope that Brahimi could find a peaceful solution.
Brahimi has had notable successes in his long diplomatic career, including helping negotiate an end to Lebanon's civil war as an Arab League envoy.
Last month, he called on the Security Council to overcome its divisions over Syria, warning that the country is "being destroyed bit by bit" with dire repercussions for the rest of region.
The three Western permanent Security Council members —Britain, France and the United States—have backed Syria's armed opposition and pushed for U.N. resolutions that threaten sanctions against Assad's regime. The other two permanent members, Russia and China, have vetoed such resolutions three times.