(Reuters) — OPEC has raised its forecast for oil supply from non-member countries in 2018 as higher prices encourage U.S. shale drillers to pump more, offsetting an OPEC-led deal to clear a supply glut and a deepening plunge in Venezuelan production.

In a monthly report on Thursday, the Organization of the Petroleum Exporting Countries (OPEC) said outside producers would boost supply by 1.15 million bbls/d this year, up from 990,000 bbls/d expected previously.

"Higher oil prices are bringing more supply to the market, particularly in North America and specifically tight oil," OPEC said in the report, using another term for shale.

OPEC, Russia and several other non-OPEC producers began to cut supply a year ago to get rid of a global glut of crude that had built up since 2014. They have extended the pact until the end of 2018.

The OPEC forecast of higher rival supply could add to a debate about the effectiveness of keeping the curbs in place. A ministerial monitoring panel meets this weekend in Oman and is expected to discuss the eventual exit strategy from the deal.

But the forecast was balanced by figures in the report showing OPEC's compliance with the supply cuts remained high in December and a further sharp slide in Venezuelan oil output.

In a further sign excess supply is easing, OPEC said inventories in developed economies declined by 16.6 million bbls in November to 2.933 billion bbls, 133 million above the five-year average.

OPEC's stated goal is to reduce stocks to the five-year average.

Venezuela, UAE

OPEC's production in December based on figures it collects from secondary sources showed overall production rising.

Total output rose by 42,000 bbls/d to 32.42 million, led by a gain in Nigeria which along with Libya was exempted from the supply cut because unrest had curbed their production.

But adherence by the 11 OPEC members with output targets rose to 129 per cent, according to a Reuters calculation based on the OPEC figures, higher than 121 per cent in November based on last month's report.

The figures that OPEC members reported themselves showed deeper declines in production.

Venezuela, whose output is dropping amid an economic crisis, told OPEC its production sank by about 216,000 bbls/d to 1.621 million bbls/d in December, believed to be the lowest in decades.

The UAE, which lagged many of its peers on compliance last year, said it cut output by 38,000 bbls/d to below its OPEC target for the first time. The compliance improvement comes as the country prepared to assume the rotating OPEC presidency in 2018.

With outside producers expected to pump more, OPEC in the report cut its estimate of global demand for its crude in 2018 by 60,000 bbls/d to 33.09 million.

Should OPEC keep pumping at December's level and other things remain equal, the market could move into a deficit of about 670,000 bbls/d next year, suggesting inventories will be drawn down further.

Last month's report pointed to a similar deficit of about 700,000 bbls/d.