US State Department spokesman John Kirby said Assad's role will be determined in the upcoming talks, but reiterated that the Syrian president must leave as a precondition for any credible peace process.
Hours earlier, US President Barack Obama said during a trip to Manila that he cannot "foresee a situation in which we can end the civil war in Syria while Assad remains in power".
Underlining the international fallout from Syria's conflict, France, still reeling from the attacks orchestrated by Islamic State extremists on Friday that claimed 129 lives in the worst terror attack on its soil, asked the United Nations to ramp up the fight against the jihadists.
In a draft resolution presented to the 15-member UN Security Council, it called on UN member states to "redouble and coordinate their efforts to prevent and suppress terrorist acts" committed by the Islamic State organisation and other extremist groups linked to Al-Qaeda.
The draft does not provide any legal basis for military action but France hopes it will rally support for the campaign against IS jihadists as it steps up its own attacks in Syria and Iraq.
"The exceptional and unprecedented threat posed by this group to the entire international community requires a strong, united and unambiguous response from the Security Council," French Ambassador to the UN Francois Delattre said.
- Start defeating terrorism' -
A General Assembly committee also passed a Saudi Arabia-sponsored resolution condemning human rights violations in Syria and calling for war crimes perpetrators to face trial.
International efforts to end the Syrian crisis have gathered steam as IS militants have carried out increasingly ambitious attacks against foreign targets and thousands of people fleeing the country have started pouring into Europe.
Russia redoubled its air strikes in Syria after the IS group claimed to have brought down a Russian jet over Egypt last month, and Foreign Minister Sergei Lavrov said Moscow would be ready to work with the Western coalition if they "respect Syria's sovereignty".
But Assad's future remains a key sticking point between Russia and Western powers, who rejected a separate Russia-backed UN resolution for international cooperation against IS extremists with the consent of the Syrian regime.
The United States and its allies argue it will be impossible for the two-year plan hashed out in Vienna, which would see a transitional government set up, a new constitution written in six months and election 18 months later, to work without clarity on when Assad will go.
His allies Russia and Iran, however, argue the president's fate should be decided by the Syrian people, and the beleaguered strongman has rejected any transition while opponents control much of the country.
"This timetable starts after starting defeating terrorism," he told Italy's Rai television channel, referring to both the jihadist groups and Western-backed rebel groups operating in Syria.
"If we talk after that, one year-and-a-half to two years is enough for any transition."
- Air strikes against IS -
IS territory across eastern, central and northern Syria is the target of a US-led air coalition as well as Russian strikes.
Air strikes on fuel trucks in the de facto IS capital, Raqa, killed at least six civilians and wounded 20, said the Syrian Observatory for Human Rights.
Observatory head Rami Abdel Rahman said those killed were oil smugglers and their families, not jihadists.
Another eight people were killed and 20 wounded in government shelling of an olive press in Sheikh Maskin, in the southern province of Daraa, the Observatory said.
Meanwhile, Syria's army and rebels struggled to pursue talks to reach a 15-day ceasefire in the Eastern Ghouta rebel stronghold east of Damascus.
The two sides had been locked in talks overnight in the hopes of reaching a deal by 6:00 am (0400 GMT), in what would be the first temporary truce in Eastern Ghouta.
After hours of relative quiet on Thursday morning, Syria's armed forces resumed shelling Douma, killing 12 people and wounding 70, the Observatory said.
A Syrian security source said "the window to reach an agreement has not ended, but we have yet to reach the results stage".