TOKYO, Jan 17 (Reuters) - Oil prices rose on Monday, with Brent crude futures at their highest in more than three years, as investors bet supply will remain tight amid restrained output by major producers with global demand unperturbed by the Omicron coronavirus variant. Brent crude futures gained 42 cents, or 0.5%, to $86.48 a barrel by 0022 GMT. The contract touched its highest since Oct. 3, 2018 - $86.71 - earlier in the session. U.S. West Texas Intermediate crude was up 62 cents, or 0.7%, at $84.44 a barrel, after hitting $84.78, the highest since Nov. 10, 2021, earlier in the session. The gains followed a rally last week when Brent rose 5.4% and WTI climbed 6.3%. Frantic oil buying, driven by supply outages and signs the Omicron variant won"t be as disruptive as feared for fuel demand, has pushed some crude grades to multi-year highs, suggesting the rally in Brent futures could be sustained a while longer, traders said. read more "The bullish sentiment is continuing as (producer group) OPEC+ is not providing enough supply to meet strong global demand," said Toshitaka Tazawa, an analyst at Fujitomi Securities Co Ltd. "If (investment) funds increase allocation weight for crude, prices could reach their highs of 2014," he said. The Organization of the Petroleum Exporting Countries (OPEC) and its allies - OPEC+ - are gradually relaxing output cuts implemented when demand collapsed in 2020. But many smaller producers can"t raise supply and others have been wary of pumping too much oil in case of renewed COVID-19 setbacks. read more Worries of a Russian attack on neighbouring Ukraine that could disrupt energy supply also lent support to prices. U.S. officials voiced fears on Friday that Russia was preparing to attack Ukraine if diplomacy failed. Russia, which has massed 100,000 troops on Ukraine"s border, released pictures of its forces on the move. read more The U.S. government has held talks with several international energy companies on contingency plans for supplying natural gas to Europe if conflict between Russia and Ukraine disrupts Russian supplies, two U.S. officials and two industry sources told Reuters on Friday. read more U.S. crude oil stockpiles, meanwhile, fell more than expected to their lowest levels since October 2018, but gasoline inventories surged due to weak demand, the Energy Information Administration said on Wednesday. read more Concerns over supply constraints outweighed the news of China"s possible oil release from reserves, Fujitomi analyst Tazawa said. Sources told Reuters China plans to release oil reserves around the Lunar New Year holidays between Jan. 31 and Feb. 6 as part of a plan coordinated by the United States with other major consumers to reduce global prices. read more