Mohammed Hamad Al-Rumhy, Oman's oil and gas minister, said Wednesday of the production cut expected at Thursday's meeting that "we haven't discussed the numbers."On Wednesday, Trump took to Twitter to urge producers to keep pumping".
His Iraqi counterpart, Thamir Ghadhban, said: "I am optimistic that the agreement will stabilize the market, will stop the slide in the price (of oil)".
The influential oil cartel is meeting at its headquarters in Vienna, Austria, with the aim of reaching an accord over production levels for the next six months.
"We are hopeful that they will come around this time and understand that everyone has to cut together", an OPEC delegate said, asking not to be named.
Donald Trump has often complained about OPEC this year as higher crude oil prices have raised fuel cost.
OPEC members are meeting to agree on their response to recent declines in oil prices.
"A massive liquidation in long positions by money managers has dampened market confidence on oil prices considerably", said Benjamin Lu of Singaporean brokerage Phillip Futures.
His comments were echoed by his Nigeriancounterpart.
Opec's de facto leader, Saudi Arabia, has indicated it wants the organisation and its allies to curb output by at least 1.3 million bpd, or 1.3% of global production.
Oil prices were pressured by a weekly report from the American Petroleum Institute (API) that said USA crude inventories rose by 5.4 million barrels in the week to Nov 30, to 448 million barrels, in a sign that U.S. oil markets are in a growing glut.
OPEC meets in Vienna on Thursday to weigh production and on Friday will hold talks with allies such as Russian Federation.
"We must be excluded from any decision to cut production as long as illegal USA sanctions are not lifted", Iran's Oil Minister Bijan Namdar Zanganeh said on Wednesday evening as he arrived in Vienna.
Iran is suffering both as a result of lower prices and a decline in its oil exports because of the U.S. sanctions.
"The oil cut will be between 1.0 and 1.3 million bpd".
One issue that might complicate relations between the USA and OPEC is the murder of Saudi Arabian journalist Jamal Khashoggi by people linked to the government in Riyadh.
The Saudi energy minister stressed that he doesn't support any exemptions from further slashing of oil output.
Otherwise, he added, the United States official has adopted an unprofessional, naïve, and meddlesome approach.
Prices have already plunged more than 30% over the past two months and a barrel of Brent crude, the European benchmark, now stands at around $60.
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