Washington: The United States sharply toughened its line on Syria, promising rebels weapons for the first time, after saying it had evidence the regime had used chemical weapons, a charge Damascus dismissed on Friday as "lies."
The harder US line dismayed Moscow, which had been working with Washington to organise a peace conference to try to end the 27-month conflict, and which described as "unconvincing" the intelligence on alleged use of the nerve agent sarin.
It also prompted expressions of concern from neutral countries such as Sweden, which warned of the risks of an arms race between the rival foreign supporters of the regime and the rebels.
President Barack Obama`s administration announced late on Thursday that it had reviewed intelligence reports and concluded that Syrian regime forces had used banned weapons, including sarin, 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 rebel Syrian Military Council.
"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.
Britain and France, which had already said publicly that they believed the Syrian government had resorted to use of its chemical weapons stockpiles, welcomed the US announcement.
But Damascus dismissed the US accusation as "a statement full of lies" and asked how Washington could continue to be regarded as an honest broker in UN-backed preparations for the proposed peace conference.
"The American decision to arm armed terrorist groups demonstrates... the direct involvement of the United States in the Syrian bloodbath," state news agency SANA quoted a foreign ministry official as saying.
It "raises serious questions about their good faith when it comes to finding a political solution in Syria," the official added.
The New York Times cited US officials it did not identify as saying that shipments to the rebels would include small arms and ammunition, and anti-tank but not anti-aircraft weapons.
The Wall Street Journal reported that US military proposals also include a limited no-fly zone over rebel training camps that could stretch up to 40 kilometres (25 miles) into Syria from neighbouring Jordan.
But France said such a plan would not be viable without UN authorisation, which would not be forthcoming given Russia`s strong opposition.
A senior aide to President Vladimir Putin said the US decision would hurt joint efforts since early May to organise a peace conference.
"What was presented by the Americans does not look convincing to us," Yury Ushakov told reporters.
Ushakov warned that Washington risked making the same mistakes it did in Iraq in 2003 when it led an invasion on the pretext that Saddam Hussein had weapons of mass destruction only to discover that none existed.
"I would not want to make any parallels, I would not want to believe that this data can be similar to the situation with the vial that (US) secretary of state Colin Powell brandished at the famous (UN) Security Council meeting."
He was referring to a meeting at which Powell held up a vial that he said could contain anthrax as he presented evidence of Iraq`s alleged arms programmes.
Sweden warned that the US promise of military aid to the rebels risked prolonging bloodshed that the United Nations says has killed at least 93,000 people since March 2011.
"I don`t think the way forward is to get an arms race going in Syria. There`s a risk that that would undermine the conditions for a political process," Foreign Minister Carl Bildt told Swedish news agency TT.
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 said the increased involvement of Lebanese Shiite militant group Hezbollah in the conflict had "added an element of urgency" to calls for a tougher response from the United States and its allies.
Gaza`s Hamas prime minister Ismail Haniya denied reports the Palestinian Islamist movement too had fighters taking part in the conflict.
The US announcement came with the rebel coalition under extreme pressure on the battlefield from loyalist forces supported by Hezbollah fighters.
The Syrian Observatory for Human Rights reported fierce battles on Friday in the northern city of Aleppo, Syria`s once-thriving commercial capital, as well as its third-largest city Homs.
Loud explosions rocked the Old City of Homs, as well as its Bab Hod neighbourhood, as the army pressed a siege they have kept up on the two rebel districts for the past year.
Nationwide, violence killed at least 106 people on Thursday, 44 of them civilians, the Observatory said.