Fighting around the Syrian capital and the airport has intensified in recent days as rebels press a battle they hope will lead to the collapse of Assad's regime after 20 months of conflict. They have set their sights on the city of 1.7 million, and fighting on the outskirts is raising fears that it soon could be facing the most brutal battle of the Syrian civil war.

The rebels issued a stern warning to the regime and travelers planning to use the country's main airport, just a few kilometers (miles) south of the capital. Loss of control of the airport would be a major blow to the regime.

A fighter who is part of the push against the airport declared it a legitimate target, claiming that the regime has stationed troops and elite forces in it as well as military planes that transport ammunition.

The clashes already have forced the suspension of commercial flights over the past week, although airport officials said Friday the facility was still functioning.

Rebels said they were trying to cut military supplies to the government. Iran and Russia are widely believed to be supplying it with weapons through the airport. Over the past months, Turkey and Iraq stopped several planes coming from those two countries that were headed to Damascus and searched them.

"This would send a very strong political message to the regime, it will be a moral victory, to say the least," said the fighter, who gave only his first name, Nour, for security reasons. "The battle to cut off the regime supplies from the airport has started."

Another rebel, speaking on condition of anonymity for the same reason, said the airport is now considered a "military zone."

"We urge civilians to stay away," said the rebel, a member of the Damascus area military command involved in Friday's fighting.

An airport official said the highway leading to the facility was closed Friday because of the fighting. The officials said, however, that the airport was functioning as normal and that people were reaching it through side roads.

The official declined to be identified because he was not authorized to give official statements.

The fresh violence around the capital comes amid growing international concerns that Assad was preparing to use chemical weapons as an act of desperation.

Syria has not confirmed it has non-conventional weapons, and insists it would never use such arms against its own people. U.S. officials say intelligence suggests the government does have the weapons, and has moved some of its stockpiles in recent days.

Also on Friday, U.N. Secretary-General Ban Ki-moon renewed a call for Syria to not use chemical weapons, saying the move would amount to an "outrageous crime" against humanity.

Speaking to reporters after visiting Syrian refugee camps in Jordan and Turkey, he also called for an end to the violence.

"The slaughter in Syria must stop. The military path is a dead end. It only fills the streets with more blood," he told reporters in Turkey.

Ahmet Uzumcu, director-general of the Organization for the Prohibition of Chemical Weapons, said Friday he had written to Syria's foreign minister urging his government to ban the use of chemical weapons by signing on to the current international convention "without delay."

Syrian opposition leaders said they were concerned the government would use its chemical weapons. George Sabra, head of the Syrian National Council opposition group, called on Syrian soldiers to ignore any orders to use them.

"Any civilian or military Syrian, who implements or helps implement this crime will be held accountable as a traitor for this country and will be pursued on charges of committing genocide," he said at a press conference in Paris.

"The Syrian people will not forgive or have mercy on anyone who orders the use of weapons of destruction and anyone who participates in executing this crime," he added.

Syrian activists said government forces were firing rockets and mortars at suburbs south of Damascus Friday amid heavy clashes with rebels. Most of the fighting was taking place in the towns of Aqraba and Beit Saham near the airport.

In other violence, a car bomb exploded in the government controlled Inshaat neighborhood in the central Homs province, setting cars on fire and covering the streets with debris, activists said.

An amateur video showed Syrian soldiers rushing to the scene as people cried in shock. It was not immediately clear what the target was.

Associated Press writers Albert Aji in Damascus, Suzan Fraser in Ankara and Mike Corder in the Netherlands contributed to this report.