The expected arrival of High Negotiations Committee (HNC) general coordinator Hijab comes as the biggest diplomatic push to date to end the almost five-year-old war hangs by a thread, with formal talks yet to begin in earnest.
His arrival was seen as a potentially positive sign, with representatives of President Bashar al-Assad's government complaining that one of the reasons the talks cannot start is what it calls disarray among the HNC.
"This is a very complicated process and it's going to require all the actors to remain in constant dialogue," the diplomat said.
Hijab, Assad's prime minister for a few months in 2012 before defecting, was chosen in December as general coordinator of the HNC, a group formed the same month in Riyadh with Saudi and US backing.
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Damascus, and its political and military backer Russia, strongly object to the inclusion within the HNC of armed groups, some of them hardline Islamist groups, seeing them as "terrorists".
UN special envoy Staffan de Mistura said on Monday that indirect negotiations had begun as foreseen in a November roadmap agreed by the many outside powers embroiled in the brutal war.
But chief government negotiator Bashar al-Jaafari yesterday said the talks were "still in the preparatory stage" and the HNC cancelled a planned meeting with de Mistura yesterday afternoon.
The HNC expressed outrage at a regime offensive near Aleppo backed by what it called "unprecedented" air strikes by Russian jets, with monitors reporting some 320 raids since Monday morning.