Iraqi officials say they have stopped pumping oil from key southern oilfields because of violence in the region.
Attacks on oil targets are costing Iraq millions
A Southern Oil Company official said militants loyal to Shia cleric Moqtada Sadr had threatened to target oil installations in the city of Basra.
An Iraqi oil ministry official confirmed the shutdown, but said any attacks on the oil industry would only harm the interests of the Iraqi people.
About 90% of Iraq's oil exports go through Basra.
"Pumping from the southern oilfields to storage tanks at Basra was stopped today after threats made by Sadr," the official told the Reuters news agency.
"It will remain stopped until the threat is over."
The official said there was enough oil in stock at Basra to continue exports from there for another two days.
Correspondents say the shutdown could badly hamper the country's reconstruction programme.
The price of oil has again started to edge up, in part because of the announcement in Iraq.