The sabre-rattling comments by Ayatollah Ali Khamenei, Iran’s Supreme Leader, at a rally in Tehran on Sunday, helped to drive US light crude prices up by a further 77 cents in afternoon trading in London, on the back of steep gains of $1.99 on Friday.

Yesterday’s continued price rises pushed the cost of a barrel of US light crude back above $73 a barrel, while benchmark London Brent added a further 98 cents to $72.01 a barrel.

Ayatollah Khamenei’s remarks dealt a blow to hopes for a compromise between Iran and the West over its nuclear programme, coming just days before today’s presentation of Western negotiating proposals to Tehran by Javier Solana, the European Union’s foreign policy chief. “If you make any mistake, definitely shipment of energy from this region will be jeopardised. You have to know this,” the ayatollah said.

His comments were a departure for Iran, which has previously said that it would not use oil as a political weapon, despite controlling a quarter of the world’s crude exports and having the capacity to close the Straits of Hormuz in the Gulf.

The Iranian threat comes at a sensitive moment for oil markets, with estimates suggesting little more than two million barrels a day of spare capacity and the industry bracing itself for possible supply disruptions from the hurricane season in the Gulf of Mexico.

Despite this, the White House joined Condoleezza Rice, the US Secretary of State, yesterday in urging calm.

President Bush’s spokesman said that Iran’s threat was “theoretical” and called for patience to allow Tehran to consider the incentive package to be offered today by Señor Solana on behalf of America, Britain, France, Germany, Russia and China in return for Iran halting its nuclear work.

“He threatened that in the case of a US invasion; that was a theoretical statement,” Tony Snow, the White House press secretary, said. “Let’s give it time, let the Iranians take a look at what the offers are, the incentives and disincentives. One can probably expect there will be a couple of quick rejoinders. We counsel patience.”

Dr Rice also responded by urging a “wait and see” stance.

Geopolitical tensions over energy also surfaced once again in Russia, meanwhile, as Gazprom, the state energy monopoly, sounded a warning that Ukraine was not pumping enough gas into underground storage. Gazprom said that this could lead to new cuts in Russian supplies to Europe for a second consecutive year.

• Lukoil, the biggest Russian oil producer, plans to double its production to four million barrels of oil equivalent per day in the next ten years, Leonid Fedun, vice-president of the group, said yesterday.

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