Syrians trying to topple President Bashar al-Assad meet their Western backers on Sunday while fighting has continued despite the Syrian government saying the year-long revolt is over.
The political opposition remains divided and has not yet formally accepted a peace plan brokered by United Nations-Arab League special envoy Kofi Annan.
Prospects of Western-led military intervention are close to zero, although Saudi Foreign Minister Prince Saud al-Faisal renewed calls on Saturday to arm the Syrian opposition, describing it as a "duty".
Assad, whose foreign ministry has declared that the revolt has been crushed, has said he accepts Annan's plan but has to keep security forces in cities to maintain security.
His opponents say they will not put down their arms until his troops and heavy weapons withdraw.
Opposition activists said security forces killed 25 Syrians on Saturday in shelling and raids on opposition areas.
A protest singer in Kafr Ruma was killed when his house was raided. A young man and his sister were shot dead when state forces stormed their village, and a man died of gunshot wounds inflicted during a protest in Damascus.
In a television address on the eve of the "Friends of Syria" meeting in Istanbul, Turkish Prime Minister Tayyip Erdogan sounded pessimistic about the chances of Assad complying with the peace plan.
"We want the attacks on civilians to stop and legitimate demands of the Syrian people to be met. Unfortunately, the Assad regime's actions do not leave any space for hope," Erdogan said.
The conference was to bring together Assad's opponents and the foreign ministers of allied Western powers, including U.S. Secretary of State Hillary Clinton.
The Annan proposal for ending what began in March 2011 as peaceful protests against Assad's rule says the army must stop violence immediately and be the first to withdraw its forces. It does not call on Assad to step down, as the opposition and its Western and Arab supporters have demanded.
Washington and Gulf Arab states, who believe Assad is simply playing for time, urged Annan to set a timeline for "next steps" if there was no ceasefire as his plan requires.
"Given the urgency of the joint envoy's mission, (U.S. and Gulf ministers) urged the joint envoy to determine a timeline for next steps if the killing continues," a statement said.
Burhan Ghalioun, president of the opposition Syrian National Council (SNC) said: "We expect immediate action and bold decisions to put an end to these crimes committed daily against the Syrian people."
"The Friends of Syria and us are preparing to save the Syrian people from real annihilation."
Ghalioun met Erdogan and senior diplomats in Istanbul on Saturday. He said they did not expect "the Assad regime will implement any item in the Annan plan and the international community will have to go the Security Council very soon, maybe after days".
The conference is expected to seek a clear endorsement of the plan from the SNC and demand that Assad order an immediate ceasefire and open two-hour daily windows for humanitarian aid.
It is not expected to recognize the SNC as the sole legitimate government of Syria, or to back arming the rebels - though Ghalioun said he hoped for support for the Free Syrian Army.
If Assad fails to keep his word, Annan will have to decide whether to tell the United Nations he has failed to make peace through a "Syrian-led process", diplomats said.
Annan will brief the Security Council on Monday on whether he sees any progress in implementing his six-point plan.
The next steps could include a return to the Security Council for a binding resolution, with increased pressure on Assad's allies Russia and China, which have endorsed Annan's mission, to get tough with Damascus.
Russia warned that it was not up to the "self-styled Friends of Syria" to decide whether Assad is keeping his word.
The U.N. peacekeeping department will send a team to Damascus soon to begin contingency planning for a possible observer mission to monitor any eventual ceasefire, Western diplomats have said.
The idea, which Annan has suggested to the Syrian government, is to have a mission of 200 to 250 observers who would be borrowed from other U.N. missions already deployed in the Middle East and Africa, envoys said. A spokesman for the U.N. Department of Peacekeeping Operations did not have an immediate comment.
More than 9,000 people have been killed by Assad's forces during the revolt, according to the United Nations, while Damascus says it has lost about 3,000 security force members.
Despite continuing violence, Damascus claims the upper hand.
"The battle to topple the state is over," Syrian Foreign Ministry spokesman Jihad al-Makdissi told Syria TV late on Friday. He said Annan had acknowledged the government's right to respond to armed violence during the ceasefire phase of the peace plan.
"When security can be maintained for civilians, the army will leave, he said. "This is a Syrian matter."
A rebel spokesman, Lieutenant Colonel Qassim Saad al-Din, told Reuters by telephone from Homs: "We don't have a problem with the ceasefire. As soon as they remove their armored vehicles, the Free Syrian Army will not fire a single shot."