The international peace conference in Geneva, Switzerland, proposed by Washington and Moscow six weeks ago to be held as soon as May is now likely to be delayed until September, a Western official told several news organizations after the close of the Group of Eight summit in Northern Ireland.
The delay report came as a G8 leaders statement at the summit's end called for the talks to be held "as soon as possible," without giving a date or even a time frame.
Those talks, as originally proposed, would seek to convince the Assad regime and the opposition to accept a six-point plan for a negotiated end to the conflict.
The plan, first laid out in Geneva in June 2012 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, increasing militant rebel force and stubborn U.N. Security Council divisions that torpedoed his efforts.
The G8 statement Tuesday called for "agreement on a transitional governing body with full executive powers" and included language that suggested Assad regime members could survive in a new government that excluded him, British Prime Minister David Cameron said.
The language said public services in Syria "must be preserved or restored" in a transition government and specified, "This includes the military forces and security services."
"For those who have been loyal to Assad but who know he has to go and who want stability in their country, they should take note of this point," Cameron said in a post-summit news conference.
Maintaining Syria's security forces after the battle is over is a lesson the West learned after the Iraq war, when disbanding the army and police left a vacuum al-Qaida exploited, Cameron added.
The G8 communique called on both the Assad regime and the opposition "to commit to destroying and expelling from Syria all organizations and individuals affiliated with al-Qaida and any other non-state actors linked to terrorism."
The al-Nusra Front jihadi group, linked to al-Qaida, has increasingly infiltrated rebel ranks and has claimed responsibility for some of the most aggressive actions of the war, including suicide bombings and attacks that have led to heavy civilian casualties.
Fighters from Lebanon's Shiite Islamic militant group Hezbollah are fighting alongside and providing other military support to Assad troops.
Both Nusra and Hezbollah are designated terrorist organizations by Washington.
Putin's insistence he would not back a peace conference convened on the assumption Assad would step down fueled fears the Russian leader was playing for time on Assad's behalf, calculating that by late summer Syria's fragmented opposition would be further weakened by military reverses, The New York Times said.
Russia is Syria's principal arms supplier.
Putin warned other G8 members during the summit against supporting U.S. plans to begin sending light arms and ammunition to Syrian rebels. Even before President Barack Obama announced his decision Thursday, France and Britain supported an end to a European embargo on arms transfers to the opposition.
The war has so far killed about 93,000 people, including women and children, the United Nations estimated last week. More than 4.25 million people have been displaced since the conflict started in March 2011.