LONDON- OPEC+ compliance with oil cuts fell slightly to 115% in September, sources said, indicating that as the alliance raises production targets, some members are still falling short as they face challenges in pumping more oil.

The Organization of the Petroleum Exporting Countries and allies led by Russia, or OPEC+ as the alliance is known, raised its output targets by 400,000 barrels per day (bpd) in September.

It has also agreed to raise them by a further 400,000 bpd in October and in November.

Underinvestment and maintenance problems have stymied efforts by Angola and Nigeria to raise output, an issue that is expected to continue impacting the West African producers in the near future. 

Last week, Saudi Arabia, the defacto leader of OPEC, defended the policy of gradual production increases from OPEC+ despite calls from major consumers like the United States to add more barrels as oil prices rise.

Asked about calls OPEC+ to increase production further, Saudi energy minister Prince Abdulaziz bin Salman: "I keep telling people we are increasing production."

Brent crude prices were trading near $86 a barrel on Monday, a three-year high, buoyed by strong demand. 

The International Energy Agency in its monthly report last week said OPEC+ spare capacity could fall to below 4 million bpd in the fourth quarter of 2022 from 9 million bpd in the first quarter of 2021. 

Spare capacity will be concentrated in Middle East producers Saudi Arabia, the United Arab Emirates, Iraq and Kuwait, the IEA said.

OPEC+ meets next on Nov. 4 to set policy for December.

(Reporting by Ahmad Ghaddar and Alex Lawler in London and Olesya Astakhova in Moscow, Editing by Louise Heavens and David Evans) ((Ahmad.Ghaddar@thomsonreuters.com; +442075424435; Reuters Messaging: ahmad.ghaddar.thomsonreuters.com@reuters.net))