Syrian rebels fighting to topple President Bashar al-Assad today declared Damascus International Airport a "legitimate target" in a bid to cut off regime supplies, as clashes between government troops and rebels forced the closure of the airport road for the second time this week.

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U.S. Secretary of State Hillary Clinton told reporters in Dublin that her meeting with Russia's foreign minister and the UN peace envoy to the Arab country was just the beginning of a difficult process to achieve peace in Syria. (Kelvin Boyes/Associated Press)

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 people, and fighting on the outskirts is raising fears that it soon could be facing the most brutal battle of the Syrian civil war.

Friday's announcement is a stern warning to the regime and travellers planning to use the airport.

Nour, a fighter who only gave his first name for security reasons and 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 in the past week, although airport officials said Friday the facility was still functioning.

'Outrageous crime' to use chemical weapons, says UN chief

UN Secretary General Ban Ki-moon has renewed a call on Syria not to use chemical weapons, saying their use would amount to an "outrageous crime in the name of humanity."

Ban on Friday also called for an end to the violence to allow a "political dialogue," and urged the UN Security Council to "stand united and act decisively" to end the crisis.

Ban spoke to reporters after a visit to refugee camps in Jordan and Turkey for Syrians who have fled the violence. He said the number of refugees has tripled since July, and a serious escalation in violence will increase numbers even further.

More than 446,000 refugees have registered in neighbouring countries, according to the UN, and 1.2 million people — nearly half of them children — are internally displaced.

Rebels said they were trying to cut military supplies to the regime. Iran and Russia are widely believed to be supplying the regime 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.

Peace talks amid chemical weapon fears

Following word of the latest concerns in Syria, U.S. Secretary of State Hillary Clinton told reporters in Northern Ireland that the U.S. and Russia have agreed to support mediation efforts with all parties in Syria to bring about a political transition.

Clinton met Thursday with Russia's foreign minister and the UN peace envoy to the Arab country, Lakhdar Brahimi, for peace talks — as the international community worries over Syria's chemical weapon possibilities.

Clinton said it was "a very constructive session" in which the trio reviewed "the very dangerous developments inside Syria."

Syria is believed to have hundreds, if not thousands, of tonnes of chemical agents, including mustard gas, a blistering agent, and the more lethal nerve agents Sarin and VX, experts say. International intelligence found Syria is moving their chemical agents, sparking concern over Assad's intentions.

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.

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UN Secretary General Ban Ki-moon visited Friday a refugee camp near Syria's border in Jordan. Ban said the number of refugees has tripled since July as more Syrians flee the violence. (Muhammad Hamed/Reuters)

"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.

UN Secretary General Ban Ki-moon said if Syria uses chemical weapons, it would be an "outrageous crime in the name of humanity" with huge consequences.

Clinton said her meeting was "just the beginning" in the difficult task of planning "a new and better future" for all of Syria's people.

"A future of this kind can not possibly include Assad," she added.

Syria's uprising began with peaceful protests in March 2011 and later escalated into a civil war that the opposition says has killed more than 40,000 people.

With files from CBC News