Oil prices have been rising since the OPEC nations, as well as other producers including Russia, decided to restrict output last year.
Last November they agreed to extend those cuts until the end of 2018.
President Donald Trump has said that unless European allies fix what he has called “terrible flaws” in the accord by May 12, he will restore US economic sanctions on Iran.
The other nations that signed the deal – France, Germany, the UK, Russia and China – all want to keep in place the agreement, which has halted Iran’s nuclear program in return for most international sanctions being lifted.
Oil price has fallen below $35 a barrel for the first time in 11 years.
Brent crude sank by 4.2% to $34.88 a barrel, surpassing its late December fall, and taking the price to its lowest level since July 1, 2004.
The price of US crude dropped 3.3% to $34.77 a barrel.
The sharp falls followed a short-lived rally on January 4 after Saudi Arabia severed diplomatic ties with Iran.
Analysts said fears over the worsening relations between Saudi Arabia and Iran, which had initially raised concerns about possible supply disruptions and boosted the oil price, had now been overtaken by pessimism over oil cartel OPEC ever agreeing on a production ceiling.
Historically, OPEC has cut production to support prices. But led by Saudi Arabia, by far the group’s most powerful member, the group has resolutely refused to trim supply this time.
Rising tensions over Saudi Arabia’s execution of Shia cleric Sheikh Nimr al-Nimr mean that any agreement is now deemed less likely than ever.
Since mid-2014, oil prices have slumped 70% mainly because of oversupply. This in turn is largely due to US shale oil flooding the market.
At the same time, demand has fallen because of a slowdown in economic growth in China and Europe.
Iranian oil exports are also expected to rise later this year once Western sanctions against Tehran for its nuclear program are lifted, increasing the oversupply of oil.
OPEC is hoping that refusing to cut production will help to drive US shale producers out of business, believing that they will fall victim to lower prices long before its own members, and has forecast that prices will recover to $70 a barrel by 2020.
Goldman Sachs has warned that oil prices could go as low as $20 a barrel, but most analysts are expecting the price to stabilize in the second half of the year as supply from non-OPEC nations slows and demand remains relatively robust.