Opec agreed on Thursday to extend oil output cuts until the end of 2018 as it tries to finish clearing a global glut of crude while signalling it could exit the deal earlier if the market overheats.

Non-Opec Russia, which this year reduced production significantly with Opec for the first time, has been pushing for a clear message on how to exit the cuts so the market doesn't flip into a deficit too soon, prices don't rally too fast and rival US shale firms don't boost output further.

The producers' current deal, under which they are cutting supply by about 1.8 million barrels per day (bpd) in an effort to boost oil prices, expires in March.

Iranian Oil Minister Bijan Zanganeh told reporters the Organization of the Petroleum Exporting Countries had agreed to extend the cuts by nine months until the end of 2018, as largely anticipated by the market.

Opec also decided to cap the output of Nigeria and Libya at 2017 levels without deciding on figures, he added. Both countries have been previously exempt from cuts due to unrest and lower-than-normal production.

Before the meetings, Saudi Energy Minister Khalid al-Falih said it was premature to talk about exiting the cuts at least for a couple of quarters and added that OPEC would examine progress at its next regular meeting in June.

"When we get to an exit, we are going to do it very gradually ... to make sure we don’t shock the market," he said.

The Iraqi, Iranian and Angolan oil ministers also said before Thursday's meetings that a review of the deal was possible in June in case the market became too tight.

But Zanganeh said later no such debate had taken place at the Opec meeting. Two Opec sources confirmed the group had agreed no concrete plan for reviewing the deal in June.

International benchmark Brent crude rose more than 1% on Thursday to trade near $64 per barrel.

With oil prices rising above $60, Russia has expressed concerns that an extension for the whole of 2018 could prompt a spike in crude production in the United States, which is not participating in the deal.

Russia needs much lower oil prices to balance its budget than OPEC's leader Saudi Arabia, which is preparing a stock market listing for national energy champion Aramco next year and would hence benefit from pricier crude.