US Secretary of State Hillary Clinton has said that nobody should have any illusions about how hard it will be to find a solution to the crisis in Syria.

Hillary Clinton held talks yesterday with Russian Foreign Minister Sergei Lavrov and the United Nations Syria envoy Lakhdar Brahimi in Dublin.

Mrs Clinton and Mr Lavrov committed to supporting a renewed push by Mr Brahimi for a political transition agreed under the Geneva Declaration in June.

"It was an important meeting but just the beginning," Mrs Clinton said at a news conference in Northern Ireland.

She described the developments on the ground as "very dangerous" for Syria and its neighbours.

"I don't think anyone believes that there was some great breakthrough, no one should have any illusions about how hard this remains but all of us, with any influence, need to be engaged with Brahimi for a concerted, sincere push."

"The United States stands with the Syrian people in insisting that any transition process result in a unified, democratic Syria in which all citizens are represented ... A future of this kind cannot possibly include Assad."

Mr Lavrov suggested there was only a slim chance of finding a solution to the crisis after the talks, according to comments published today.

Meanwhile, Syrian rebels fighting to topple President Bashar Assad declared Damascus International Airport a "legitimate target" in a bid to cut off regime supplies.

Clashes reported between government troops and rebels forced the closure of the airport road for the second time this week.

Fighting around the Syrian capital and the airport has intensified in recent days.

Rebels are pressing a battle they hope will lead to the collapse of President Assad's regime after 20 months of conflict.

They have set their sights on the city of 1.7m 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 travellers planning to use the country's main airport.

The airport is located just a few kilometres south of the capital.

Loss of control of the airport would be a major blow to the regime.

Meanwhile, Syria has not confirmed it has non-conventional weapons, and insists it would never use such arms against its own people.

US officials say intelligence suggests the government does have the weapons, and has moved some of its stockpiles in recent days.

Also today, UN 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.