"We were hoping that we'd be in a position to announce a date today, unfortunately we're not," UN-Arab League peace envoy Lakhdar Brahimi told reporters in Geneva on Tuesday.
"But we're still hoping that we'll be able to have the conference before the end of the year," he said, adding he would meet again with Russian and US officials on November 25.
His comments followed a day of intense discussions with senior diplomats from the world powers in Geneva aimed at preparing a new international conference to try to end the Syrian conflict.
But Tuesday's meeting ended without a date.
Pressed to reveal the main stumbling blocks, Brahimi cited divisions within the Syrian opposition, saying he hoped they would "move towards the formation of a credible delegation" for peace talks.
"That will be a big step forward to make the conference possible," the Algerian veteran international mediator said.
The main umbrella opposition National Coalition has said it plans to meet in Istanbul on Saturday to decide whether to attend the peace talks.
But a key member of the bloc has threatened to quit if it decides to attend, and some rebels have warned all participants will be deemed traitors.
The fate of President Bashar al-Assad is crucial to the opposition which has insisted that his resignation be on the negotiating table -- a demand rejected by Damascus.
A senior US official voiced optimism that the conference, dubbed Geneva II, could be held by the end of the year, but stressed the importance of giving the splintered opposition time to create a representative delegation.
"This is about building the future of Syria. That is a long, difficult, complicated process. If it takes the opposition coalition a few more weeks to prepare themselves in the way they feel they need to .. we want to support them to do that," the official said, requesting anonymity.
Russia, a staunch ally of the Syrian regime, was meanwhile planning to meet with members of the Syrian opposition in Geneva on Wednesday, Deputy Foreign Minister Gennady Gatilov, who took part in Tuesday's talks, told Russian media.
US scepticism over chemical weapons
The momentum for a new peace conference came after a landmark Russian-US accord reached in September to destroy Syria's chemical arsenal by mid-2014, raising hopes that diplomatic efforts might also manage to coax the warring sides to the table later this month.
But on the same day as the discussions in Geneva, the US ambassador to the United Nations voiced "scepticism" about Syria's declaration of its chemical weapons to an international watchdog and said it was checking its accuracy.
"More work of course remains to be done to ensure that the Syrian government's list of declared sites is comprehensive and that the process remains on track, particularly as we enter the destruction phase," said the US envoy Samantha Power.
"We obviously bring scepticism born of years of dealing with this regime...", she added.
The Syrian regime for its part has been expected to participate in the proposed Geneva peace conference, but on Monday it stressed it would not accept negotiations that aimed to oust Assad.
"We will not go to Geneva to hand over power as desired by (Saudi Foreign Minister Prince Saud) al-Faisal and certain opponents abroad," Syrian information minister Omran al-Zohbi was quoted by the official SANA news agency as saying.
Visiting Poland Tuesday, US Secretary of State John Kerry underscored that the point of a peace conference was to put in place a transition government acceptable to both sides.
"I don't know how anybody believes the opposition is going to give mutual consent to Assad to continue," Kerry told reporters, a day after visiting Riyadh to try to calm Saudi fears that Washington's resolve to push Assad from power is fading.
Saudi Arabia and Qatar, two major backers of the Syrian rebels, have meanwhile stressed the need for a clear timetable for the conference.
Tuesday's meeting took place amid growing alarm about the humanitarian situation in Syria, where more than 120,000 people have been killed in the 31-month conflict.
Some 9.3 million people, or nearly half of the Syrian population, are now in need of humanitarian assistance while 6.5 million people have been displaced in Syria and some 2.2 million have fled the country, according to UN data.