The United States dramatically toughened its line on Syria, accusing it of using chemical weapons and promising military aid to rebel forces, as Damascus on Friday slammed the allegation as "lies".
President Barack Obama's administration announced Thursday it had reviewed intelligence reports and concluded that Syrian regime forces had used banned arms, including sarin nerve gas, in attacks that killed up to 150 people.
US officials refused to rule out moving towards arming rebels or imposing a no-fly zone, and said Washington would provide backing to the Syrian Military Council (SMC) armed opposition.
"The president has made a decision about providing more support to the opposition. That will involve providing direct support to the SMC. That includes military support," deputy national security adviser Ben Rhodes said, declining to elaborate.
Syria dismissed the US accusation as "a statement full of lies about the use of chemical weapons in Syria, based on fabricated information".
The Wall Street Journal reported that US military proposals include a limited no-fly zone over rebel training camps.
This zone would stretch up to 40 kilometres (25 miles) into Syria, and would be enforced by warplanes inside Jordan airspace armed with long-distance air-to-air missiles, the Journal reported, citing unnamed US officials.
The New York Times cited unnamed American officials as saying weapons for the rebels would include small arms and ammunition and anti-tank weapons but not anti-aircraft weapons.
Syria's main opposition National Coalition said in a statement issued by its Washington office that it "welcomes increased US assistance including direct military support".
"The support should be strategic and decisive in order to force an end to the violence and to achieve a political transition," it said.
Washington's decision will hurt the chances of a new Russia-US peace initiative on the crisis, a senior foreign policy aide to Russian President Vladimir Putin said.
"What was presented by the Americans does not look convincing to us," Yury Ushakov told reporters.
"The information that has been presented, the facts that have been presented do not look convincing to us," he said.
The chances of holding a Syrian peace conference that Russia and the United States proposed jointly on May 7 would be hurt by the decision to provide military support for the opposition, he added.
British Foreign Secretary William Hague said in a statement London agreed with the US assessment of chemical weapons use in Syria.
"The crisis demands a strong, determined and coordinated response from the international community," he said.
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NATO Secretary General Anders Fogh Rasmussen welcomed the "clear" US statement, saying Damascus should grant UN access "to investigate all reports of chemical weapons use".
The European Union also said the allegations reinforced the need for UN inspectors to be deployed in Syria, and for increased efforts to find a political solution to the conflict.
Hawkish US lawmakers welcomed the Obama administration's change in position but Senator John McCain said the president needed to go further.
"We need heavy weaponry. We need the kind that can counter tanks, and we need surface-to-air missiles," McCain said.
Rhodes did not confirm weapons would be sent but warned Washington had toughened its stance.
He said the increased involvement of the Lebanese militia Hezbollah and Syria's ally Iran in the conflict had "added an element of urgency" to calls for a tougher response from the United States and its allies.
Military experts have long warned that a no-fly zone would require jets to destroy Syrian air defences, but US planners believe it could be imposed in about a month without having to do that.
A US defence official had previously told AFP Washington will keep F-16 fighters and Patriot anti-missile batteries in Jordan after a joint exercise ends this month.
The military also plans to keep US Marines on amphibious ships after consultations with Jordan's leadership, he said.
Washington has long led demands, echoed by its European and Arab allies, that Assad must step down before Syria's factions can begin to reunite the country after more than two years of civil war that has left tens of thousand of people dead.
But it has resisted calls to arm the divided rebels amid fears that many are Islamist extremists with ties to Al-Qaeda and like-minded extremist anti-Western groups.
Thursday's US announcement came with the rebel coalition under extreme pressure on the battlefield from loyalist forces supported by Iran-backed militiamen from Lebanon.
Fighting raged on Friday, with the Syrian Observatory for Human Rights reporting fierce battles in the northern city of Aleppo, Syria's once-thriving commercial capital.
It said it was unclear if the fighting was part of a larger assault the regime has vowed to unleash on Aleppo and the surrounding province, large parts of which are under rebel control.
With the conflict escalating, the United Nations in Geneva said at least 93,000 people, including more than 6,500 children, have been killed.
The UN also said in a statement from Damascus that more than 1.2 million people there were "in urgent need of humanitarian assistance" in Damascus province.