However, Syrian President Bashar Assad and top regime officials "cannot play any role in the future of Syria," the officials said in a statement released by the U.S. State Department.
It said the formation of a transitional governing body had been identified as the "corner stone of a political solution." It called for such a body to be set up within a specific timeframe that would have "full executive authority."
That transitional government would be tasked with adopting a new constitution for Syria "that guarantees the equal rights of all citizens," the statement said.
U.S. Secretary of State John Kerry said Washington and allies might provide weapons and other support to Syria's rebels if diplomacy fails to end the civil war.
"In the event that we can't find that way forward, in the event that the Assad regime is unwilling to negotiate in good faith, we will also talk about our continued support and growing support for the opposition in order to permit them to continue to fight for the freedom of their country," Kerry said in Amman, Jordan, before a Friends of Syria meeting with key Western and Arab nations backing the rebels.
Kerry lowered prospects for an international peacemaking conference in Switzerland that include representatives of the opposition and Assad's regime.
"These are big stakes and I don't have any illusions about how difficult it is to find that path forward," Kerry said.
The Geneva meeting, proposed two weeks ago by the United States and Russia, which back opposing sides in the conflict, would take place next month under U.N. auspices. It would seek to get both sides to accept a six-point plan for a negotiated end to the conflict laid out in Geneva 11 months ago.
The June 2012 plan, proposed by special joint U.N.-Arab League peace envoy Kofi Annan, called for a transitional government and is widely considered the most serious international attempt to resolve the Syrian civil war diplomatically.
Annan, whose peace plan was all but ignored by the warring parties, quit in August, saying he was frustrated by Syrian regime intransigence, an increasing rebel militancy and stubborn U.N. Security Council divisions that torpedoed his efforts.
As for next month's conference, Syria's opposition has refused to commit to attending, while the regime has said it was open to the possibility.
"We are always willing to cooperate and collaborate with any opposition that has a program that serves the interest of Syria," Syrian Ambassador to Jordan Bahjat Suleiman said in Amman Wednesday before Kerry's arrival.
The opposition Syrian Observatory for Human Rights said this month the civil war death toll was about 120,000. The United Nations estimated the death toll in February as 70,000.