Share markets slide as oil price falls below $30
Global share markets tumbled as the prospect of an end to the Iranian oil export ban sent oil below $30 a barrel for the second time this week.
Wall Street opened down sharply, with the Dow Jones index sinking 2.33% to 15,997.94 points.
It came as concern grew that Iran could restart oil exports, flooding an already over-supplied market.
The oil benchmark Brent crude fell 4.7% to $29.43.
US West Texas intermediate oil fell 5% to $29.51.
Shares in mining firm Anglo American were the worst hit, falling by more than 11%. Glencore was down 7%, and BHP Billiton and Antofagasta lost 6%.
Rangold Resources rose by 5%. Gold prices tend to do well in times of investment uncertainty. The gold price held steady with a gain of 0.5% at $1093.75 an ounce.
Iran could restart exports if the International Atomic Energy Agency (IAEA) reports it has complied with measures to curb its nuclear programme.
The IAEA could publish its report as early as Friday, following a meeting in Vienna.
Iran has the fourth largest proven oil reserves in the world, according to the US Energy Information Agency and any additional oil would add to the 1 million barrels a day of over supply that has led to a more than 70% collapse in oil prices since the middle of 2014.
"With sanctions on Iran likely to be lifted, more oil is flooding the markets," Commerzbank analysts wrote in a note.
"Although the additional supply had been imminent for some time, current sentiment ought to send prices further south."
Commerzbank cut its 2016 forecast for oil prices, changing its year-end expectation for Brent to $50 per barrel, down from a previous forecast of $63.
Iran's oil exports were already on target to hit a nine-month high in January, with 1.10 million barrels a day of crude, to load.
Why is the oil price so low?
Oil prices have fallen by about 70% in the past 18 months as supply has outstripped demand. The demand for oil from China has fallen as its economic growth has slowed. Meanwhile supply has increased, partly due to the rise of US shale oil. In addition, the world's largest exporter of oil, Saudi Arabia, has refused to cut production - something it has done previously to support oil prices. Analysts estimate that about one million barrels of oil are being produced above demand every day.
Consumers and some businesses have benefitted from lower oil prices. UK motorists have seen the price of petrol and diesel fall from about £1.40 a litre 18 months ago to about £1 now. Transport operators and airlines should also be benefitting from cheaper fuel. The lower fuel costs have also helped to keep inflation close to zero in many countries.
Oil exporting nations that rely on a higher oil price to break even are suffering, such as Russia, Nigeria and Venezuela, as are oil firms generally. There have been thousands of job losses in the North Sea's oil industry. Investment in exploration has also been cut by big oil firms such as Shell, BP, Total and Exxon Mobil.
Iran is expected to target India, Asia's fastest-growing major oil market, as well as its old partners in Europe, such as Greece, with the increased exports.
It is the wrong time for Iran to be returning to the oil market, both for the market and likely also for Iran," brokers Phillip Futures said in a note on Friday.
The continuing collapse in oil prices has also added to pressure on oil producing nations that rely on exports including Algeria, Venezuela, Nigeria and Russia.
He said that the country must be prepared for a "worst-case" economic scenario if the price continued to fall.
Taxes from oil and gas generates about half the Russian government's revenue.
The 2016 federal budget that was approved in October was based on an oil price of $50 a barrel in 2016 - a figure President Vladimir Putin has since described as "unrealistic".