A US pledge to step up military aid to Syrian rebels on the basis of alleged proof the regime had used chemical arms drew criticism Friday from UN chief Ban Ki-moon and Russia.
Syrian rebel fighters hold a position on June 13, 2013 in the northwestern town of Maaret al-Numan. The United States has dramatically toughened its line on Syria, accusing it of using chemical weapons and promising military aid to rebel forces, as Damascus slammed the allegation as "lies".
Damascus itself dismissed the accusation as "lies".
The UN secretary general said that arming either side in the 27-month Syrian conflict that has cost tens of thousands of lives "would not be helpful".
Ban told reporters he has been "consistently clear that providing arms to either side would not address this current situation. There is no such military solution."
The harder US line also dismayed Moscow, which had been working with Washington to organise a peace conference.
US data on the Syrian regime's alleged use of chemical weapons was "unconvincing", it said, warning Washington against repeating the mistake it made when invading Iraq after falsely accusing Saddam Hussein of stocking weapons of mass destruction.
The Kremlin's top foreign policy adviser Yury Ushakov also said the US decision to provide military aid to Syrian rebels would damage international efforts to end the conflict.
Russian President Vladimir Putin and US President Barack Obama are to meet at the G8 summit in Northern Ireland on Monday.
The US decision to up the ante 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.
Obama's administration announced late Thursday that it had reviewed intelligence reports and concluded that 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 it as "a statement full of lies" and asked how Washington could continue to be regarded as an honest broker in any UN-backed 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.
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," 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 US response.
The US announcement came with the rebel coalition under extreme pressure on the battlefield from government forces supported by Hezbollah fighters.
In a televised address on Friday, Hezbollah leader Hassan Nasrallah said his group will stay involved in the conflict, after having helped the regime recapture the key town of Qusayr.
"Before Qusayr is the same as after Qusayr. Nothing has changed," he said.
"Isn't the conspiracy the same?... Have the facts changed? On the contrary, the other side is stirring up this conflict even more," said the leader of the powerful Iranian-backed group
In the latest fighting, the Syrian Observatory for Human Rights reported, fierce battles swept the northern city of Aleppo, Syria's once-thriving commercial capital and its third-largest city Homs.
Loud explosions rocked the Old City of Homs and its Bab Hod neighbourhood, as the army pressed a siege they have kept up on the two rebel districts for the past year.