Washington ? Oil prices marched to new highs just shy of $45 a barrel Monday after an Iraqi oil official said output was stopped for security reasons at key southern oil fields. While oil in storage was still being loaded onto tankers, analysts feared the market could lose more than 1.5 million barrels a day by the end of the week.

Buying also fueled by the renewed threat of a supply disruption in Russia, analysts said, as the embattled oil giant Yukos was dealt more bad news.

Light crude for September delivery rose by 89 cents to $44.84 per barrel on the New York Mercantile Exchange. On London’s International Petroleum Exchange, Brent crude futures gained 93 cents to $41.56 per barrel.

“I think to a large extent the market has factored in the fact that Iraqi production is going to go in and out,” said Peter Beutel, president of the energy consulting firm Cameron Hanover Inc. in New Canaan, Conn. “But supplies are so tight and demand keeps growing.”

The Organization of Petroleum Exporting Countries has had difficulty soothing markets lately, as traders remain skeptical of the cartel’s ability to quickly boost output.

A senior official with Iraq’s oil company, who spoke on condition of anonymity, said the southern oil fields stopped pumping oil Monday after militants loyal to Shiite cleric Muqtada al-Sadr threatened to target the oil infrastructure in Basra, a southern port.

Iraqi Oil Ministry spokesman Assem Jihad said he could not confirm the shutdown. Neither could the British military, which patrols Basra.

About 1.8 million barrels per day, or 90 percent of Iraq’s exports, move through Iraq’s southern port of Basra.

If production is shut down, Iraq has enough oil in storage tanks to maintain exports at today’s levels for another three days or so, according to John Kingston, director of oil at Platts, a division of McGraw-Hill Cos.

The Al Shaeba oil refinery is shown on the outskirts of the southern port city of Basra. Iraq stopped pumping oil from its key southern oil fields Monday because of the violence plaguing the region during a renewed Shiite uprising, an official with the oil refinery said.

Global oil demand is roughly 82 million barrels a day, or about 4 percent higher than a year ago, according to PFC Energy, a Washington-based consulting group.

Last week, concern about Russian supplies was the dominant factor in oil markets.

The Russian government is in a battle with oil giant Yukos over billions of dollars in back taxes, and company officials have said its production of 1.7 million barrels per day could suffer.

On Monday, the embattled oil company Yukos suffered a double blow as bailiffs renewed their seizure of assets of its key production unit and a court rejected the company’s appeal against the seizure of another subsidiary.

Yukos has warned that it could face bankruptcy as early as this month if the state does not give it access to frozen bank accounts and sells off the Yuganskneftegaz subsidiary, its main cash earner.