Rebel commanders reached by phone said their fighters had advanced to within a few hundred yards of the airport perimeter. Syria’s state-run media, which have portrayed the Aleppo fighting by insurgents as a futile effort by criminal gangs, inadvertently confirmed the insurgent advance, reporting that government troops deployed around the airport had repulsed attacks.
“Our fighters are in all neighborhoods close to the airport,” said a rebel commander who identified himself as a former air force pilot named Wasel. The commander, who did not provide his full name for security reasons, also said the insurgents were benefiting from replenished supplies of ammunition after chronic shortages, “which is a reason for this progress.”
The Aleppo fighting, as well as heavy clashes reported by activists in the Damascus area, came as the United Nations and the Arab League announced a successor for Kofi Annan, the special Syria envoy who resigned in frustration two weeks ago over his inability to halt the conflict, now in its 18th month. Mr. Annan’s successor, Lakhdar Brahimi, a former Algerian foreign minister and veteran diplomat who helped broker the end of Lebanon’s civil war, is expected to take up his new role in coming days.
Mr. Brahimi’s appointment was announced in a brief statement by Ban Ki-moon, the United Nations secretary general.
“The violence and the suffering in Syria must come to an end,” Mr. Ban said in the statement. “The secretary general appreciates Mr. Brahimi’s willingness to bring his considerable talents and experience to this crucial task, for which he will need, and rightly expects, the strong, clear and unified support of the international community, including the Security Council.”
Mr. Ban’s statement appeared to be a diplomatic swipe at the Security Council, which has been deeply divided over how to deal with Syria since the political uprising against President Bashar al-Assad began. Russia and China, permanent Council members with veto power, blocked efforts by the other members to threaten the Syrian government with coercive measures over Mr. Assad’s harsh repression of protesters and his failure to implement Mr. Annan’s peace plan as promised.
Mr. Annan, who called his job “Mission: Impossible,” attributed his failure in Syria partly to his inability to bring such pressure to bear.
Mr. Brahimi, who has done other troubleshooting work for the United Nations, including in Afghanistan and Iraq, did not immediately comment on his appointment. But he issued a statement last week deploring the violence in Syria and calling on all Syrians to embrace tolerance as part of a new formula for peace.
“In the meantime, the U.N. Security Council and regional states must unite to ensure that a political transition can take place as soon as possible,” Mr. Brahimi said in the statement, which was issued in his capacity as a member of the Elders, a group of accomplished international leaders that also includes Mr. Annan.
The appointment of Mr. Brahimi was announced a day after the Security Council decided to end the United Nations observer mission in Syria, which was created in March as part of Mr. Annan’s peace plan. Disregard for the plan by Mr. Assad’s forces and the rebels seeking to topple him made the presence of the observers irrelevant and endangered their safety.
President Assad has found it increasingly difficult to portray an image of confidence and control, undermined by a rash of defections and improvements in the fighting ability of the insurgents, who are getting military assistance from Turkey, Qatar and Saudi Arabia, and now occupy pieces of territory along the Turkish border. They have tied down Syrian military units in Aleppo, Damascus and other major cities.
The highest-ranking defector so far, Riyad Farid Hijab, the former prime minister, visited Qatar on Friday for what his spokesman said were discussions on “unifying the efforts of the opposition to accelerate the pace of the downfall of the regime,” Reuters reported.
Mr. Assad’s authority has also been weakened by an increasingly urgent humanitarian crisis in Syria, where, according to the United Nations, an estimated 2.5 million people need aid, more than one million have been uprooted from their homes and tens of thousands have fled to neighboring countries, mostly Turkey.
“Fighting continues in and around other cities in addition to Aleppo, including Homs, Damascus, Deir al-Zour, Idlib and Dara’a,” Marianne Gasser, the head of the International Committee of the Red Cross delegation in Damascus, said in a statement on its Web site. “We are very concerned about the effects that the fighting is having on civilians in these areas.”
In a new threat, the World Health Organization reported that contamination in the water supply of rural Damascus from sewage had caused an outbreak of diarrhea. The Red Cross said its aid teams were working on water systems in Aleppo, rural Damascus, Deir al-Zour and Homs. NYT