Syria and key ally Russia joined forces on Wednesday against any Western-backed United Nations resolution that would allow military action, as Moscow accused UN chemical weapons inspectors of bias.
The United States, meanwhile, said it will maintain the threat of force if Damascus fails to abide by an accord to surrender its chemical arsenal, and the United Nations hit back at the Russian accusations.
Moscow came out swinging, saying Damascus had delivered new proof implicating rebels in an August 21 sarin gas attack near the capital that killed hundreds, while slamming a UN report into the incident.
Syrian President Bashar al-Assad, hosting Russian Deputy Foreign Minister Sergei Ryabkov, thanked Moscow for its support throughout his regime's 30-month-old conflict with rebels.
He "expressed... his gratitude to Russia for its position of helping Syria face down the savage attack... and the Western, regional and Arab-backed terrorism," state television quoted him as saying.
His comments came as UN chemical weapons inspectors confirmed they would be returning to Syria to investigate other alleged cases of the use of the deadly weapons.
Russia and the United States continued to trade accusations about who was to blame for the sarin attack that the weapons inspectors confirmed in a report published this week.
Despite having jointly agreed a deal under which Syria will turn over its chemical weapons stockpile, the two nations remain at loggerheads over who launched the attack.
The US military will maintain the threat of force against Syria, US Defence Secretary Chuck Hagel said.
"We should keep that military option exactly where it is. We have assured the president that our assets and force posture remain the same," he told a news conference.
Russia said the Syrian regime had supplied new evidence implicating the rebels in the deadly incident and that this would be given to the United Nations.
But US President Barack Obama has said it was "inconceivable" that anyone other than the Syrian regime could have carried out the attack.
Chapter VII resolution
The international community is also divided over the wording of a UN Security Council resolution on the US-Russia deal, with Moscow strongly opposing a Chapter VII resolution.
France and Britain are reportedly preparing a draft resolution including a demand for action under Chapter VII -- which allows the use of force and tough sanctions -- if Syria fails to uphold the deal on its chemical weapons.
But Russia has said there was "no basis" for a UN resolution to invoke Chapter VII.
According to Ryabkov, Syria has handed over new evidence showing it was opposition forces were behind the sarin attack.
"Without a full picture... we cannot describe the character of the conclusions as anything other than politicised, biased and one-sided," Russian news agency RIA Novosti quoted him as saying.
He accused the inspectors of ignoring "very factual" evidence provided by Damascus.
Evidence related to the deadly August 21 incident "was given to Mr (Ake) Sellstrom who headed the group of UN inspectors", he said in remarks aired on Russian television.
Ryabkov also said the Syrian regime gave assurances it will comply with the one-week deadline to hand over information about the size and location of its chemical weapons, in line with the US-Russia deal.
In New York, UN chief Ban Ki-moon's spokesman rebuffed the Russian charges and said the secretary general had the "fullest confidence in the professionalism of his team and their work and findings.
"They have worked impartially and to the highest scientific standards despite the exceptionally difficult conditions of the war in Syria. They will continue to do so."
The five UN Security Council powers held new talks Wednesday on a resolution backing the Russia-US plan to destroy Syria's chemical weapons, diplomats said.
Western nations, who say they are not looking for an immediate threat of force against Assad's regime, could seek a Security Council vote this weekend if agreement can be reached with Russia.
Sellstrom, meanwhile, told AFP that his team would return to Syria to investigate additional alleged attacks.
The deal under which Syria will turn over its chemical weapons came after Washington threatened military action against Damascus in response to last month's chemical attack.
The deal has suspended talk of military action, but NATO Secretary General Anders Fogh Rasmussen said Wednesday that the threat should remain to ensure Syria keeps its word.
In the latest fighting, an Al-Qaeda front group overran the northern town of Azaz near the border with Turkey after fierce clashes with rebels, activists told AFP.
"The Islamic State of Iraq and the Levant (ISIS) has seized complete control of Azaz. They are in control of the town's entrances," said Abu Ahmad, an activist inside the town.
Elsewhere, at least four regime air raids hit Barzeh in northern Damascus, according to the Syrian Observatory for Human Rights.
And it said Kurdish fighters had forced jihadists to withdraw from a village in the northeast after clashes killed 21 on both sides.
The conflict that broke out in March 2011 has killed more than 110,000 people, according to rights groups, and several million refugees have flooded countries in the region and beyond.