Oil prices fell on Monday as as Russia signaled its output will remain high and concern over the global economy put crude on track for its biggest monthly fall since mid-2016.
Losses were limited ahead of U.S. sanctions on Iranian exports that are expected to reduce supplies when they come into effect in just under a week.
U.S. crude futures ended Monday's session down 55 cents to $67.04. The contract has slumped about 8.5 percent in October, on pace for its largest monthly percentage decline since July 2016.
Brent crude oil futures were down 8 cents at $77.54 a barrel at 1:38 p.m. ET, dropping about 6.3 percent this month and also threatening to post the worst decline since July 2016.
Even with U.S. sanctions on Iranian exports due to come into force on Nov. 4, oil prices have fallen about $10 a barrel since four-year highs reached in early October.
Russian Energy Minister Alexander Novak said on Saturday there was no reason for Russia to freeze or cut its oil production levels, noting that there were risks that global oil markets could be facing a deficit.
The OPEC, led by Saudi Arabia and non-OPEC member Russia, agreed in June to lift oil supplies, but OPEC then signaled last week that it may have to reimpose output cuts as global inventories rise.
"When the Russians start talking about keeping the production levels high and even the possibility that they need to increase it because of a possible tightness in supply, that brought on some selling pressure," said Gene McGillian, director of market research at Tradition Energy in Stamford, Connecticut.
Industrial commodities such as crude and copper have been rattled by hefty losses in global equities due to concern over corporate earnings, and fears over the impact to economic growth from escalating trade tensions.
Though equity markets recovered on Monday, the benchmark was on track to post its worst monthly performance since May 2010.
The U.S. dollar index also rose, supported by robust U.S. consumer spending data. A stronger dollar makes greenback-denominated commodities more expensive for holders of other currencies.
"It is often said that when stock markets sneeze, commodities catch a cold. This adage was on full display last week as a global rout on equity gauges dragged the energy complex lower," PVM Oil Associates strategist Stephen Brennock said.
Fund managers have cut their bullish positions in crude futures and options for four weeks in a row to their lowest since July 2017, as the demand outlook grows more uncertain.
On the supply side, Iran has started selling crude to private companies via a domestic exchange for the first time, the oil ministry's news website reported.
With just days to go before U.S. sanctions on Iran take effect, three of Iran's top five customers — India, China, and Turkey — are resisting Washington's call to end oil purchases outright, arguing there are not sufficient supplies worldwide to replace them, according to sources familiar with the matter.
That pressure, along with worries of a damaging oil price spike, is raising the possibility of bilateral deals to allow some buying to continue, according to the sources.
— CNBC's Tom DiChristopher and Gina Francolla contributed to this report.