By Parisa Hafezi and Erika Solomon
ANKARA/BEIRUT, June 14 (Reuters) - The United States is considering a no-fly zone in Syria, potentially its first direct intervention into the two-year-old civil war, Western diplomats said on Friday, after the White House said Syria had crossed a "red line" by using nerve gas.
After months of deliberation, President Barack Obama's administration said on Thursday it would now arm rebels, having obtained proof the Syrian government used chemical weapons against fighters trying to overthrow President Bashar al-Assad.
Two senior Western diplomats said Washington is looking into a no-fly zone close to Syria's southern border with Jordan.
"Washington is considering a no-fly zone to help Assad's opponents," one diplomat said. He said it would be limited "time-wise and area-wise, possibly near the Jordanian border", giving no further details.
Imposing a no-fly zone would require the United States to destroy Syria's sophisticated Russian-built air defences, thrusting it into the war with the sort of action NATO used to help topple Muammar Gaddafi in Libya two years ago. Washington says it has not ruled it out, but a decision is not "imminent".
"We have not made any decision to pursue a military operation such as a no-fly zone. And we have a range of contingency plans that we've drawn up," U.S. Deputy National Security Adviser Ben Rhodes said on Thursday.
"A no-fly zone would carry with it great and open-ended costs for the United States and the international community. It's far more complex to undertake the type of effort, for instance, in Syria than it was in Libya."
Any such move would also come up against a potential veto from Assad's ally Russia in the U.N. Security Council. The Kremlin dismissed U.S. evidence of Assad's use of nerve gas.
"I will say frankly that what was presented to us by the Americans does not look convincing," President Vladimir Putin's senior foreign policy advisor Yuri Ushakov said. "It would be hard even to call them facts."
France said a no-fly zone would be impossible without U.N. Security Council authorisation, which made it unlikely for now.
Nevertheless, Washington has quietly taken steps that would make it easier, moving Patriot surface-to-air missiles, war planes and more than 4,000 troops into Jordan in the past week, officially as part of an annual exercise but making clear that the assets could stay on when the war games are over.
Syria's civil war grew out of protests that swept across the Arab world in 2011, becoming by far the deadliest of those uprisings and the most difficult to resolve, with powers across the Middle East squaring off on sectarian lines.
Western countries have spent the past two years demanding Assad leave power but declining to use force as they did in Libya, because of the far greater risk of fighting a much stronger country that straddles sectarian divides at the heart of the Middle East and is backed by Iran and Russia.
Just months ago, Western countries believed Assad's days were numbered. But momentum on the battlefield has turned in his favour, making the prospect of his swift removal and an end to the bloodshed appear remote without outside intervention.
Thousands of seasoned fighters from Lebanon's pro-Iranian Hezbollah militia joined the war on Assad's behalf in recent weeks and last week helped the Syrian government recapture Qusair, a strategic town. Assad's government says its troops are now preparing for a massive assault on Aleppo, Syria's biggest city, mainly in rebel hands since last year.
Activists reported an intensified assault on parts of Aleppo and its countryside near the Turkish border overnight, sparking some of the most violent clashes in months.
The use of chemical weapons provides a straightforward reason for Washington to intervene. Deputy National Security Adviser Rhodes said Washington now believed 100-150 people had been killed by government poison gas attacks on rebels.
"The president ... has made it clear that the use of chemical weapons or transfer of chemical weapons to terrorist groups is a red line," he said. "He has said that the use of chemical weapons would change his calculus, and it has."
Syria, which says rebels used chemical weapons not the government, said the U.S. statement was full of lies.
"The White House ... relied on fabricated information in order to hold the Syrian government responsible for using these weapons, despite a series of statements that confirmed that terrorist groups in Syria have chemical weapons," the foreign ministry said in a statement.
An implicit threat to join the conflict puts Washington on a diplomatic collision course with Moscow, which has used its U.N. Security Council veto three times to block resolutions that might be used to threaten force against Assad.
U.S. officials say Obama will try to persuade Putin to abandon support for Assad when the two leaders meet at a G8 summit in Northern Ireland next week.
Washington and Moscow have jointly called for a peace conference in Geneva, the first attempt in a year by the Cold War foes to find a diplomatic solution to the war, but the prospects for the talks now seem in doubt.
The United Nations now estimates at least 93,000 people have been killed in Syria and millions driven from their homes.
The arrival of thousands of Iran-backed Shi'ite Hezbollah fighters to help Assad combat a revolt led by Syria's Sunni majority has shifted momentum and raised the prospect of sectarian violence spreading across the Middle East.
Western powers have been reluctant in the past to arm the rebels, worried about the rising strength of Sunni Islamist insurgents among them who have pledged loyalty to al Qaeda. European countries like France say the best way to counter such Islamists is to provide more support for mainstream rebels.
The White House said Washington would now provide "direct military support" to the opposition. A U.S. official, speaking on condition of anonymity, confirmed it would now include arms as opposed to "non-lethal" aid sent in the past.
That puts once-reluctant Washington a step in front of its allies Paris and London, which have forced the European Union to drop a ban on arming the rebels this month but still say they have not yet taken a decision to send arms.
Syrian rebels already receive light arms from Saudi Arabia and Qatar. They have asked for heavier weapons including anti-tank and anti-aircraft missiles.
U.S. and European officials are meeting the commander of the rebels' Supreme Military Council, Salim Idriss, on Friday in Turkey.
Qassem Saadedine, a Supreme Military Council commander, called Obama's decision to send weapons "very brave".
"Our hope is that the weapons will start arriving in the coming weeks, but we are still in talks about when and how to supply weapons. My hope is we will start seeing a change in the next two weeks."
Islamist rebel fighters on the ground in Syria were more sceptical. "All of us inside Syria know the truth is America hates Sunni Muslims," said Abu Bilal, a Sunni insurgent in Homs.
"We consider America an enemy and see it as quite unlikely that it will actually give the mujahideen weapons. Instead it will be preparing its own agenda, so that it can hit the rebels just like it will hit the regime," he told Reuters via Skype.
An Islamist field commander in Hama said he would take the weapons if he could get them: "Everyone here right now is working on the principle that their enemy's enemy is their friend. America is against Bashar right now, at least publicly."
"As for us, we will look at the issue this way: we do not object to groups that take weapons from America. We will object to those who try to spread its secular ideas in Syria."
FIGHT FOR ALEPPO
Assad forces tried overnight to storm the rebel-held eastern districts of Aleppo, Syria's biggest city and commercial hub, according to the Syrian Observatory for Human Rights, a Britain-based, pro-opposition monitoring group.
The move sparked some of the fiercest battles in months. Activists also reported artillery and air strikes in the rebel-held countryside in the north of the province.
Syrian state media have been touting plans for "Northern Storm," a looming campaign to recapture the rebel-held north.
Aleppo would be a far more difficult target than Qusair. Assad's forces only hold a few routes and pockets of territory in the province, mostly in isolated Shi'ite villages.
Assad's main advantage so far has been the ability to use air power to resupply troops and bomb rebel areas, along with its use of long-range missiles. But Western support for rebels or a no-fly zone would change the current balance of power. (Additional reporting by Erika Solomon in Beirut, John Irish in Paris; Khaled Yacoub Oweis in Amman, Matt Spetalnick, Roberta Rampton, Mark Felsenthal, Jeff Mason and Susan Heavey in Washington; Writing by Peter Graff and Erika Solomon; editing by Janet McBride)