Rebels and activists in Syria say anti-regime forces are advancing in Aleppo and have taken control of several neighborhoods

Activists in Aleppo say there are heavy clashes around the city, particularly around a military airport. A rebel commander Bassam al-Dada told The Associated Press on Thursday that anti-regime fighters have taken two central areas in the country's largest city, including Salaheddin, where battles have raged for months. Al-Dadda is an adviser to rebel leader Riad al-Asaad.

Rebel forces are advancing in the city's other neighborhoods, al-Dada said.

THIS IS A BREAKING NEWS UPDATE. Check back soon for further information. AP's earlier story is below.

The deputy head of the United Nations warned Thursday that there are no guarantees that a proposed Syrian cease-fire will hold, but urged Syrian rebels and the regime in Damascus to observe it.

The 15-nation U.N. Security Council unanimously endorsed the idea of a four-day cease-fire proposed by the U.N. secretary-general with the aim of setting up talks on ending the country's 19-month-old conflict. The truce is set to start Friday, during the Muslim holy day of Eid al-Adha.

A day before the proposed truce, U.N. Deputy Secretary-General Jan Eliasson said, "We all have our eyes on the tragedy in Syria, and we pin our hopes now on the cease-fire that hopefully can take place."

Eliasson, a former Swedish foreign minister, urged both sides on the battlefield to seize the opportunity to ratchet down the bloodshed and create a climate for conducting talks.

"We very much hope that this first step towards the reduction of violence and the beginning of the political progress will be taken because we see very great dangers, both vis-a-vis the Syrian people and the future of the nation of Syria, and of course also for the security and stability of the region," he told reporters in Geneva, adding that there are many signs the conflict is spreading beyond Syria.

Lakhdar Brahimi, the U.N.-Arab League envoy for Syria, has warned that the failure of yet another U.N. cease-fire plan would only worsen the fighting.

A "cease-fire has its major significance in the symbolic quieting, silencing of the guns, and letting the Syrian people finally have silence around themselves for the possibilities to see what the fighting has done. But the most important thing is that it could, possibly, create an environment in which a political process is possible," Eliasson said.

Eliasson said that U.N. Secretary-General Ban Ki-moon proposed during a meeting in New York with Syria's foreign minister several weeks ago that both sides lay down their arms during Eid al-Adha.

"Hundreds of thousands of people are in grave danger. We have refugee flows across the borders. Winter is approaching in Syria, and those winters are harsh," he added. "There are problems with the electricity grids. We see huge humanitarian problems ahead of us. It's already serious but it could become even worse."