WRAPUP 1-Rebels say Damascus airport is now a battle zone
* Rebels say: Approach airport "at your own risk"
* Russia plays down chances of diplomatic solution
* Clinton says events "accelerating" on the ground
BEIRUT, Dec 7 (Reuters) - Rebels fighting to topple Syrian President Bashar al-Assad declared Damascus International Airport a battle zone on Friday, warning civilians and airlines they would approach it "at their own risk".
Fighting around the capital city has intensified over the past week, prompting predictions among Western opponents of Assad that an endgame is approaching in a 20-month-old conflict that has killed 40,000 people.
"Events on the ground in Syria are accelerating, and we see that in many different ways," U.S. Secretary of State Hillary Clinton said before talks on Thursday with Foreign Minister Sergei Lavrov of Russia, which has backed Assad.
"The pressure against the regime in and around Damascus seems to be increasing," Clinton said in Dublin.
Syria's government says that is not the case, and that the army is driving rebels back from positions in the suburbs and outskirts of Damascus where they have tried to concentrate their offensive.
Many who have followed the events on the ground say talk of an endgame is overblown or premature.
"I think it's unreasonable to expect that the battle is in its last stages right now," said Rami Abdelrahman of the Syrian Observatory for Human Rights, which has tracked the fighting since it began in March 2011.
"The big advances are only in the media. The situation is certainly not good, for anyone. The Syrian economy is dead. But conditions for the rebels are not good either ... Rebel-held parts of Aleppo are barely eating and are always at risk of army shelling."
"It is true however that the regime is withdrawing from many areas... and the regime is being exhausted," he told Reuters.
Cutting access to the airport 20 km (12 miles) from the city centre would be a symbolic blow. The rebels acknowledge it is still in army hands.
"The rebel brigades who have been putting the airport under siege decided yesterday that the airport is a military zone," said Nabil al-Amir, a spokesman for the rebels' Damascus Military Council. "The airport is now full of armoured vehicles and soldiers."
"Civilians who approach it now do so at their own risk," he said. Fighters had "waited two weeks for the airport to be emptied of most civilians and airlines" before declaring it a target, he added. He did not say what they would do if aircraft tried to land. A rebel spokesman on Thursday said fighters would not "storm the airport but we will blockade it".
Foreign airlines have suspended all flights to Damascus since fighting approached the airport in the past week, although some Syrian Air flights have used the airport in recent days.
Washington and its European and Arab allies have long sought the overthrow of Assad, while Russia has shielded him at the U.N. Security Council. Signs that President Vladimir Putin may be losing patience with Assad have been detected at various points, but so far Moscow has not shifted.
Lavrov played down the chance of finding a diplomatic solution to the crisis in Syria after talks with Clinton and international envoy Lakhdar Brahimi.
"Russia and the United States agreed that our experts will meet in the coming days with Brahimi and his people for a brainstorm and an exchange of opinions on how to move forward toward a resolution," Lavrov said.
"I would not make optimistic predictions ... It remains to be seen what will come out of this," he added, noting that Brahimi knows the chance of success is "far from 100 percent".
U.S. "VERY CONCERNED" OVER CHEMICAL ARMS
The United States and its NATO allies have issued coordinated warnings in recent days to Assad not to use chemical weapons. United Nations Secretary-General Ban Ki-Moon also urged Assad not to use poison gas.
"I think there is no question that we remain very concerned, very concerned that as the opposition advances, in particular on Damascus, that the regime might well consider the use of chemical weapons," U.S. Defense Secretary Leon Panetta said.
Syria has not signed an international chemical weapons treaty banning poison gas, but has repeatedly said that it would never use such weapons on its own people.
Assad's government said the warnings about chemical arms were aimed at whipping up an excuse for military intervention.
NATO decided this week to send U.S., German and Dutch batteries of air-defence missiles to the Turkish border, meaning hundreds of American and European troops will deploy to Syria's frontier for the first time since the war began.
The United States is in no hurry to get into another Middle Eastern war. If it appeared that chemical weapons were about to be used by Assad, or seized by an extreme Islamist faction among the rebel forces, it could be driven to take action.
But some analysts say there is no long-range plan.
"Western powers have been too focused just on their endgame, which was to see Assad go," says Hayat Alvi, associate professor of national security studies at the US Naval War College. "There is no substantive plan about any other contingencies, risks, and post-Assad scenarios."