Hijab, a former Syrian premier who defected in 2012, went straight to a Geneva hotel to meet other members of his High Negotiations Committee (HNC).
A reporter said that UN envoy Staffan de Mistura also arrived at the hotel, with an opposition source telling AFP saying he was there to meet Hijab informally.
Hijab's arrival was seen as a potentially positive sign because of the weight he carries with the HNC, a Saudi-backed opposition alliance that is opposed to President Bashar al- Assad.
One such person is Mohammed Alloush, a leading member of Islamist rebel group the Army of Islam, in Geneva since Monday and nominally the HNC's chief negotiator.
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"The problem is not with de Mistura. The problem is with the criminal regime that decimates children and with Russia which always tries to stand alongside criminals," Alloush told reporters today, clutching a photo of a young boy he said was severely wounded by Russian air strikes
But this declaration proved to have been premature, with the chief government negotiator rejecting that this was the case and the HNC cancelling a meeting with de Mistura yesterday in anger at Russian air strikes in Syria.
"With Hijab here, the HNC can better demonstrate a unified position in representing the opposition," a Western diplomat said in Geneva on condition of anonymity.
"This is a very complicated process and it's going to require all the actors to remain in constant dialogue," the diplomat said.