Syria on Friday slammed Washington's claims that it had used chemical weapons in its fight against rebels, accusing the United States of "lies" based on "fabricated information".
"The White House published a statement full of lies about the use of chemical weapons in Syria, based on fabricated information, through which it is trying to hold the Syrian government responsible for such use," state news agency SANA quoted a foreign ministry official as saying.
Washington said on Thursday there was evidence that the Syrian government had used chemical weapons including sarin nerve gas in attacks that killed up to 150 people.
The US administration also said it would now provide "military support" to rebels, a decision condemned by Damascus.
The Syrian official said the US claims came "after reports affirming that armed terrorist groups active in Syria are in possession of deadly chemical weapons and the technology necessary to make them".
"The American decision to arm armed terrorist groups demonstrates... the direct involvement of the United States in the Syrian bloodbath," the official added.
And he said the US decision to arm the rebels "raises serious questions about their good faith when it comes to finding a political solution in Syria."
Washington and Moscow -- which is allied with the Syrian regime -- have been spearheading efforts to organise a conference in Geneva to find a political solution to the Syrian conflict.
But the prospects for the conference are unclear, with the opposition setting conditions for its attendance and the proposed date already pushed back at least a month to July.
"The United States is practising a flagrant double standards policy when it comes to terrorism," the official said.
"The US administration decided to designate the Al-Nusra Front on their list of terrorist groups and at the same time offer support to terrorist groups by offering them weapons, money and equipment."
Washington, he said, was also offering "political cover, by stopping the Security Council from condemning the massacres that these groups have committed".
He referred to the deaths of at least 60 Shiite residents of the village of Hatlah, where rebels carried out an assault earlier this week.
The Syrian Observatory for Human Rights said on Tuesday rebels had killed at least 60 people in the village in eastern Deir Ezzor province after armed residents attacked a rebel checkpoint a day earlier.