The pact on freezing output has helped support oil prices, which started a slide in mid-2014 because of oversupply. Brent crude was trading above US$40 a barrel yesterday, up 50 per cent from a 12-year low of US$27.10 reached in January.
Some delegates are wary that if many oil producers meet too soon before getting the nod from everyone, prices could weaken again if the talks ended in disarray.
Opec's Gulf members favour meeting in the first half of April, in Doha or another Gulf city, according to a Gulf delegate last week.
Only if Iran agrees will things change.
Tehran has rejected freezing its output at January levels, put by Opec sources at 2.93 million barrels per day (bpd), and wants to return to much higher pre-sanctions levels.
Tehran wanted a freeze based on four million barrels a day, its pre-sanctions production figure, said sources familiar with the discussions.
The issue is set to be discussed when Russian Energy Minister Alexander Novak visits Tehran on Monday. AAP