Strong demand for oil and ongoing geopolitical tensions will keep oil prices at a relatively high level for the rest of 2018 despite reports that Saudi Arabia and Russia may increase supplies, Fitch Ratings said.

The rating agency raised its oil price assumptions for 2018, with Brent likely to average $70 a barrel and the U.S. West Texas Intermediate to average $65 a barrel in 2018, compared to $57.50 and $55.00, respectively, previously.

For 2019, Fitch now expects Brent at $65.00 per barrel and WTI at $60.00, up from the previous assumption of $57.50 and $55, respectively.

The revisions reflect high year-to-date prices, Venezuela's production decline, ongoing geopolitical tensions, including the renewal of U.S. oil sanctions on Iran, and strong demand growth, Fitch said.

The OPEC's and Russia's plan to increase supplies, due to be finalized June 22, should help make up for the production declines in Venezuela and a possible decline in exports from Iran due to renewed US sanction, according to Fitch. However, any production ramp-ups are expected to be "gradual and coordinated" so as to avoid a massive surplus, according to the rating agency.

Fitch assumes a gradual price fall to below $60 per barrel for both Brent and WTI by 2020.

Fitch maintained its long-term price assumptions at $57.50 a barrel for Brent and $55 for WTI, subject to cost inflation levels and producers' ability to preserve efficiency gains achieved so far. "The U.S. shale industry should remain a marginal oil producer and should be able to meet a significant portion of global demand growth for the next several years," the rating agency said.