As a six-month-old spat between Saudi Arabia and Qatar deepens, OPEC's Gulf ministers will have to scrap their tradition of meeting behind closed doors to agree on policy before the organisation holds its twice-yearly talks, OPEC sources say.

"We used to have a WhatsApp group for all ministers and delegates from the Gulf. It used to be a very busy chat room. Now it's dead," said a senior source in OPEC, the Organization of the Petroleum Exporting Countries.

Four other sources said there had been no official contact on oil policy between the Gulf Arab nations, in a grouping known as the Gulf Cooperation Council (GCC). The GCC includes OPEC members Saudi Arabia, the United Arab Emirates, Kuwait, Qatar and non-OPEC Oman and Bahrain.

OPEC meeting

OPEC will meet on November 30 in Vienna to decide whether to extend global output cuts beyond March. OPEC kingpin Saudi Arabia and the UAE cut ties with Doha in June, saying Qatar backed terrorism and was cosying up to rival Iran. Qatar rejected the accusation.

"The ministers can't meet," another OPEC source said. "They may relay the message through the Kuwaiti or the Omani oil ministers, but Saudi and the UAE cannot meet publicly with the Qataris."

Kuwait and Oman have refrained from taking sides in the dispute, over which Kuwait's Emir Sheikh Sabah has led regional mediation.

OPEC has survived worse crises and operated under even greater strain, including the Iran-Iraq war in the 1980s, Iraq's invasion of Kuwait in 1990, as well as tensions between Saudi Arabia and Iran over the past decade.

None of the OPEC sources suggested the Qatar crisis would derail a widely expected decision by OPEC to extend price-boosting output cuts until the end of 2018, as almost all producers agree on the need to maintain policy.

But dialogue within OPEC is likely to be complicated as the standoff strikes at the heart of OPEC's efforts to form a united front to stabilise a fragile oil market.

As OPEC president in 2016, Qatar was instrumental in bringing together oil producers - including non-OPEC Russia - to agree on the supply-reduction deal.

"If the GCC is dead politically, then it will certainly have implications for OPEC policies. Not that it will necessarily disrupt decision-making, but it is making it more challenging and complicated," the senior OPEC source said.

Iraq and Iran

With the world's fourth- and fifth-largest oil reserves, Iraq and Iran are seen as the OPEC countries with the largest output growth potential and hence together can be the biggest challengers to the leading role Riyadh has played for decades.

Iraq has resisted calls from the United States to lessen its reliance on Tehran. Iran also plans to import significant volumes of Iraqi oil.

"The Saudis perfectly understand that challenge and are doing their utmost to lessen Iran's influence on Iraq," a third OPEC source said.

Relations between Riyadh and Baghdad have been improving in recent months, with the two states joining hands to coordinate their fight against ISIL and their efforts to rebuild Iraq.

Saudi Energy Minister Khalid al-Falih visited Iraq in October to call for increased economic and energy cooperation, the first Saudi official to make a public speech in Baghdad in decades.