(Adds opposition source)
BEIRUT, Feb 28 (Reuters) - Syria's political opposition has postponed talks to choose the leader of a provisional government, two opposition sources said on Thursday, hours after a meeting with Western powers produced few tangible results for President Bashar al-Assad's foes.
Opposition leaders had hoped a meeting in Istanbul on Saturday would elect a prime minister to operate in rebel-controlled areas of Syria, threatened by a slide into chaos as the conflict between Assad's forces and insurgents approaches its second anniversary.
"The meeting may be held later in the week," Mohammad Sarmini of the Syrian National Council said by telephone.
Another opposition source said the meeting was delayed indefinitely, saying the three most likely candidates for prime minister had reservations about taking on the role in the absence of more concrete international support.
News of the postponement came hours after the United States announced it would send non-lethal aid directly to Syrian rebels, disappointing Assad's opponents who are clamouring for Western weapons and more active support.
The United Nations says around 70,000 people have been killed in Syria's conflict, which erupted in March 2011 with mainly peaceful protests against Assad but has escalated into a civil war with sectarian overtones, threatening to draw in regional powers.
(Reporting by Mariam Karouny; Editing by Andrew Heavens)