Global benchmark Brent crude oil futures LCOc1 were at $83.26 per barrel at 0352 GMT, down 90 cents, or 1.1 percent, from their last close.
U.S. West Texas Intermediate (WTI) crude futures rose 67 cents to settle at $74.96 a barrel, a 0.90 percent gain.
Iranian Oil Minister Bijan Zanganeh on Monday described a Saudi claim that the kingdom could replace Iran's crude exports as "nonsense".
US Secretary of State Mike Pompeo last month said that on November 4th the sanctions against Iran will be enforced and that the US will consider waivers for countries that bring their Iranian oil purchases down to zero. An industry source who also tracks exports said October shipments so far were below 1 million bpd.
The decrease apparently was fueled by buyers seeking alternatives ahead of the start of US sanctions on November 4.
Sources said this time around the entire 100 per cent of Iranian oil import bill can be paid in rupees.
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By Navneet Damani Crude oil prices continued to rally for yet another week, as traders stayed focussed on potential disruptions to global crude supplies in the upcoming week, as looming U.S. sanctions on Iran are widely expected to lead to a tighter market.
Iran's crude exports fell further in the first week of October, according to tanker data and an industry source, taking a major hit from US sanctions and throwing a challenge to other OPEC oil producers as they seek to cover the shortfall. In September, Iran exported 1.6 million bpd.
Iran may indeed have not cut production yet to match the rate of decline in its exports, as the country appears to be storing more oil on ships as it did during sanctions that applied until the 2015 nuclear deal.
The details provided by Refinitiv Eikon showed the Islamic Republic exported 1.1 million barrels per day (bpd) of crude last week.
Oil prices have extended a rally on expectations the sanctions will test the Organization of the Petroleum Exporting Countries and other producers. While private companies like Reliance and Nayara Energy (Essar) have had to drastically reduce their purchases, public sector companies, who are more immune to USA strictures, and already have alternate banking arrangements and a "rupee-rial" mechanism, are staying the course for now. For 2018/19, the real economic growth was expected to moderate, but still, the International Monetary Fund expected Iran's economy to grow by 4 percent in 2018/19, forecasting stable oil production in line with the OPEC cap on Iran's production.
Another production headwind helping to boost prices, Hurricane Michael, is expected to hit the Gulf of Mexico region Tuesday, threatening almost 300 miles of the Gulf coast. The American Petroleum Institute is due to release data on Wednesday, while the U.S. Energy Information Administration is due to publish on Thursday.