A US pledge to step up military aid to Syrian rebels because of alleged use of chemical arms by the regime drew criticism from Damascus ally Russia and from UN chief Ban Ki-moon.
And Syria itself dismissed the accusation as "lies."
The UN secretary general said arming either side in the 27-month war, which has cost tens of thousands of lives, "would not be helpful".
Ban said 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 chemical weapons was "unconvincing", it said, warning the Washington against repeating the mistake it made when invading Iraq after falsely accusing Saddam Hussein of stocking weapons of mass destruction.
Top Kremlin 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 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 on 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.
Deputy national security adviser Ben Rhodes said "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," he said, declining to elaborate.
Britain and France, which had already said 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 as saying shipments to the rebels would include small arms and ammunition and anti-tank weapons, but not anti-aircraft weapons.
The Wall Street Journal said 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 incoming US national security adviser Susan Rice acknowledged there are "downsides and limitations" to imposing a no-fly zone.
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 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," he said.
Representatives from countries that support the Syrian opposition met in Istanbul with the SMC chief, General Selim Idriss, to discuss possible weapons deliveries, an opposition spokesman said.
Khaled Saleh, a spokeman for the opposition Syrian National Council said: "At this stage, we expect that the international community would begin to supply the SMC... with sophisticated arms, anti-tank and anti-aircraft missiles because that is what the Assad regime uses."
The US announcement came with the rebel coalition under extreme pressure on the battlefield from government forces supported by Hezbollah fighters from neighbouring Lebanon.
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.
In Cairo, meanwhile, thousands of Islamists rallied to express support for calls by several Sunni Muslim clerics for a holy war against Assad's regime.
Egypt, is an overwhelmingly Sunni Muslim country, and it is Sunnis who are the backbone of the revolt against Assad, whose Alawite sect is an offshoot of Shiite Islam.
In the latest fighting, the Syrian Observatory for Human Rights reported, fierce battles swept the northern city of Aleppo and the central city of 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.