Brent futures were down 44 cents, or 0.68pc, at $63.95 per barrel by 0935 GMT, after gaining about 3pc last week on news that OPEC and its allies would deepen output cuts.
West Texas Intermediate oil futures were down 48 cents, or 0.81pc to $58.72 a barrel, having risen about 7pc last week on the prospects for lower production from "OPEC+", which is made up of the Organization of the Petroleum Exporting Countries and associated producers including Russia.
Monday's sudden chill came after customs data released on Sunday showed exports from the world's second-biggest economy in November fell 1.1pc from a year earlier, confounding expectations for a 1pc rise in a Reuters poll.
The weak start to the week came despite data showing China's crude imports jumped to a record, revealing just how deep jitters are embedded in the market over the US-China trade row that has stymied global growth and oil demand.
"China is clearly not immune to either the US trade tariffs, or the lingering slowdown in the broader global economy," said Jeffrey Halley, senior market analyst at OANDA.
Washington and Beijing have been trying to agree a trade deal that will end tit-for-tat tariffs, but talks have dragged on for months as they wrangle over key details.
Beijing hopes an agreement with the United States can be reached as soon as possible, China's Assistant Commerce Minister Ren Hongbin said on Monday.
Monday's declines also went against signs on Friday that China was easing its stance on resolving the trade dispute with the United States, confirming that it was waiving import tariffs for some soybean and pork shipments.
The price drops also put an end to a strong run in previous sessions fuelled by hopes for the OPEC+ production curb deal.
On Friday, those producers agreed to deepen their output cuts from 1.2 million barrels per day (bpd) to 1.7 million bpd, representing about 1.7pc of global production.
"This decision crystallises an important shift in strategy to managing short-term physical imbalances rather than trying to correct perceived long-term imbalances through open-ended commitments," Goldman Sachs said in a note.