The United States is considering imposing a no-fly zone in Syria, its first direct military intervention of 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 equivocating, 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 Assad.

Two senior Western diplomats said Washington is mulling 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," without giving details.

France said on Friday that the no-fly zone was unlikely for now because of opposition from some members of the United Nations Security Council. 

Imposing a no-fly zone would require the United States to destroy Syria's air defenses, entering the two-year-old civil war with the sort of action that NATO used to help topple Muammar Gadhafi in Libya two years ago.

The area near the Jordanian border contains some of the most densely-populated parts of Syria, including the outskirts of the capital Damascus.

Washington has moved 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 forces deployed 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 take direct action like that taken in Libya, because of the far greater risk of engaging with 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 favor, making the prospect of his swift removal or 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 an 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 Ben Rhodes said Washington now believed 100-150 people had been killed by poison gas.

"Our intelligence community assesses that the Assad regime has used chemical weapons, including the nerve agent sarin, on a small scale against the opposition multiple times in the last year," he told reporters.

"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 considers the rebels terrorists. Its foreign ministry called the U.S. decision to arm the rebels a "flagrant double standard" in its dealings with terrorism.

Syria's state news agency SANA said: "the United States affirmed once again its involvement in supporting the armed terrorist groups in Syria by announcing its intention to provide them with greater military aid."

On Friday, Syria's foreign ministry said the U.S. was lying about chemical weapons use to give it an excuse to intervene in the civil war. "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," it said.

"The United States, in resorting to a shameful use of pretexts in order allow President Obama's decision to arm the Syrian opposition, shows that it has flagrant double standards in the way it deals with terrorism."

Collision course with the Kremlin

The implicit threat to openly join the conflict puts Washington on a diplomatic collision course with Moscow, which has used its UN 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 Russian President Vladimir Putin to abandon support for Assad. Obama and Putin will meet at a G8 summit in Northern Ireland next week.

The Kremlin did not immediately respond directly to the new U.S. posture but a Kremlin-allied Russian lawmaker said the chemical weapons data was fabricated and that Washington would use it to cook up a justification for joining the war.

"Information about the use by Assad of chemical weapons has been fabricated in the same place as the lies about (Saddam) Hussein's weapons of mass destruction," said Alexei Pushkov, head of the foreign policy committee in the lower chamber of the Russian parliament, in a statement on Twitter.

"Obama is taking the same path as George Bush."

U.S. officials briefed Russia on the information they had, Ushakov said at a briefing before Putin's trip to Northern Ireland for a G8 summit. "But I will say frankly that what was presented to us by the Americans does not look convincing."

"It would be hard even to call them facts," said Ushakov, speaking at a briefing before a summit where Putin will meet Obama and other Western leaders.

"If the Americans ... carry out more wide-scale aid to the rebels and opposition, it will not make organizing the international conference easier," he said.

The arrival of thousands of seasoned, 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.

The United Nations now estimates at least 93,000 people have been killed in Syria and millions driven from their homes.

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 their loyalty to the global militant network Al-Qaida.

The White House said Washington would now provide "direct military support" to the opposition. It did not publicly specify whether this would include "lethal aid," which would mark a reversal of Obama's previous resistance to arming the rebels. But a U.S. official, speaking on condition of anonymity, said the package would include weapons.

Syrian rebels already receive light arms from Saudi Arabia and Qatar. They have asked for heavier weapons including anti-tank and anti-aircraft missiles. European countries, in particular France, have argued that the solution is to provide more weapons for mainstream rebels to marginalise extremists.

U.S. and European officials are meeting the commander of a main rebel fighting force, the Free Syrian Army (FSA), on Friday in Turkey. FSA chief Salim Idriss is expected to plead urgently for more help.

Obama has been more cautious than Britain and France, which forced the European Union this month to lift an embargo that had blocked weapons for the rebels. French officials already said a week ago that they had firm proof Assad used nerve gas, while Washington continued to say it was unsure until Thursday.

Rebels demand weapons

Syrian rebel and political opposition leaders immediately called for sophisticated weaponry.

"We want anti-tank and anti-aircraft weapons," George Sabra, acting leader of the National Coalition political opposition bloc, told Al-Arabiya television. "We expect to see positive results and genuine military support."

U.S. Senator John McCain, a hawk on Syria, said America needed to neutralise Assad's air power: "They (rebels) have enough light weapons. They've got enough AK-47s. AK-47s don't do very well against tanks," McCain told CNN. "They need anti-tank weapons and they need anti-air weapons."

NATO Secretary-General Anders Fogh Rasmussen welcomed what he said was a "clear U.S. statement". "The international community has made clear that any use of chemical weapons is completely unacceptable and a clear breach of international law," Rasmussen told reporters in Brussels.

Syria is not a signatory to the international treaty that bans chemical weapons but has said that it would never use them in an internal conflict. It has accused the rebels of using them, but Washington said it does not believe that the insurgents have access to them.