UN-Arab League peace envoy Lakhdar Brahimi was set to meet top US and Russian officials on Friday to discuss the Syrian conflict, a day after Damascus denounced his "bias" against President Bashar al-Assad.
Washington meanwhile expressed concern about securing Syria's chemical weapons should Assad fall from power.
Damascus's criticism of the veteran Algerian diplomat came after he described proposals Assad made on Sunday for a "political solution" to the conflict as "one-sided".
Brahimi attacked Assad's plan to keep fighting rebel "terrorists" and ignore opposition groups tied to them, in comments to the BBC Wednesday.
He also questioned the decades-long rule by Assad's family, saying: "In Syria in particular, what people are saying is that one family ruling for 40 years is a little bit too long." Assad took over from his father in 2000.
Syria's pro-government Al-Watan newspaper denounced Brahimi as a "pawn" of the West.
A Syrian foreign ministry statement broadcast on state television Thursday attacked what it called Brahimi's "flagrant bias for those parties known to be conspiring against Syria and its people".
On Friday, Brahimi is to meet in Geneva with Russian Deputy Foreign Minister Mikhail Bogdanov and US Deputy Secretary of State William Burns to discuss the Syrian conflict.
But the scathing exchange further undermined prospects of international diplomacy working to calm what has become a civil war in Syria, with more than 60,000 deaths, according to the UN.
In Washington, Defense Secretary Leon Panetta said Thursday the United States were increasingly focused on how to secure Syria's chemical weapons if Assad falls from power.
Panetta said he would not consider sending ground troops into the war-torn country, even to secure chemical sites, but he left the door open to some US military presence if Assad's downfall was followed by a peaceful transition.
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While the US government has issued stern warnings to Damascus against resorting to chemical weaponry in its war with rebel forces, Panetta said the greater risk might be a chaotic vacuum if Assad is toppled.
"I think the greater concern right now is what steps does the international community take to make sure that when Assad comes down, that there is a process and procedure to make sure we get our hands on securing those sites," he said.
"That, I think, is the greater challenge right now."
In Britain, Foreign Secretary William Hague said world powers would have to step up their response to the conflict if the violence worsens, warning that all options were on the table.
He reiterated that Britain would seek to amend the EU weapons embargo on Syria when it comes up for review on March 1 to allow them to arm rebels opposed to Assad's regime.
Hague told members of parliament that Britain was supporting Brahimi's efforts to end the conflict, and revealed the envoy would visit London for talks later this month.
Meanwhile Assad's key ally Iran was pursuing its separate diplomatic track in support of Damascus.
Foreign Minister Ali Akbar Salehi visited Cairo on Thursday for talks on the conflict with Egyptian President Mohamed Morsi and Foreign Minister Mohamed Kamel Amr.
Any solution should be worked out among Syrians, without "foreign intervention," Salehi told reporters.
Iran has sent financial aid and Revolutionary Guard military advisors to Syria. It is the only country to have come out fully in support of Assad's plan.
Russia and China have so far blocked international action against Assad's regime in the UN Security Council.