The Syrian government has dismissed as "full of lies" claims from the United States that it used chemical weapons.
It has accused US President Barack Obama of resorting to fabrications to justify his decision to arm Syrian rebels.
The commander of the main rebel umbrella group welcomed the US move, saying it would lift his fighters' morale.
The US decision to begin arming the rebels marks a deepening of US involvement in Syria's two-year civil war.
It comes as President Bashar al-Assad's forces have been scoring victories, driving rebels out of a key town near the Lebanese border, and launching new offensives in Aleppo.
US officials said the administration could provide the rebel fighters with a range of weapons, including small arms, ammunition, assault rifles and a variety of anti-tank weaponry such as shoulder-fired rocket-propelled grenades and other missiles.
However, no final decisions have been made on the type of weaponry or when it would reach the rebels, according to the officials.
In addition to the increased military aid, the US also announced yesterday it had conclusive evidence that Mr Assad's regime has used chemical weapons, including the nerve agent sarin, on a small scale against opposition forces.
The White House said multiple chemical attacks last year killed up to 150 people.
Mr Obama has said the use of chemical weapons cross a "red line," triggering greater US involvement in the crisis.
But a statement issued by Syrian Foreign Ministry said: "The White House has issued a statement full of lies about the use of chemical weapons in Syria, based on fabricated information."
The statement also accused the US of "double standards", saying America claims to combat terrorism while providing support for "terrorist" groups in Syria, such as Jabhat al-Nusra, with arms and money.
The group, also known as the Nusra Front, is an al-Qaeda affiliate that has emerged as one of the most effective rebel factions in Syria.
The commander of the main Western-backed rebel group fighting in Syria said he hoped that US weapons will be in the hands of rebels in the near future.
Mr Assad's forces, aided by fighters from Lebanon's militant group Hezbollah, captured Qusair on 5 June, dealing a heavy blow to rebels who had been entrenched in the strategic town for over a year.
Since then, the regime has shifted its attention to recapture other areas in the central Homs province and Aleppo to the north.
The regime's advances have added urgency to US discussions on whether to provide the rebels with weapons.
The decision comes a day after the United Nations said nearly 93,000 people have been confirmed dead in Syria's civil war, but the actual number is believed to be much higher.
Russia, a staunch ally of Mr Assad, has disputed the US charge that Syria used chemical weapons against the rebels.
President Vladimir Putin's foreign affairs adviser, Yuri Ushakov, told reporters that the information provided by US officials to Russia "didn't look convincing".
The White House said Barack Obama will discuss Syria, Afghanistan and the issue of arms control with President Putin during the G8 summit.