Iran has starkly warned Gulf states not to make up for any shortfall in its oil exports under new US and EU sanctions, adding yet another layer of peril to the international showdown over its nuclear programme.

If Arab neighbours compensate for a looming EU ban on Iranian imports, "we would not consider these actions to be friendly," Iran's representative to OPEC, Mohammad Ali Khatibi, was quoted as saying by the Sharq newspaper on Sunday.

"They will be held responsible for what happens" in that case, he said, adding ominously: "One cannot predict the consequences."

The warning comes as Iran is being hammered on several fronts over its nuclear programme, which it is defiantly expanding.

Western sanctions are being ratcheted up, shaking Iran's oil-dependent economy.

Military pressure is building, with the United States and Britain deploying warships to the Gulf, and reports suggesting Israel could be poised to launch air strikes against Iranian nuclear sites.

A covert campaign has also been stepped up, as evidenced last Wednesday by the murder of a deputy director of Iran's main uranium enrichment plant -- the fifth scientist to be targeted in Tehran by motorbike-mounted assassins in two years.

Furious Iranian officials say they are looking at ways of hitting back against those they see responsible for the attacks: arch-foes the United States, Israel and Britain.

They are also threatening to close the Strait of Hormuz in the Gulf if extra sanctions bite, cutting off the transport of 20 percent of the world's oil.

The United States has said that would cross a "red line," prompting likely military action.

Iran's foreign ministry on Sunday revealed that Washington had sent Tehran a letter concerning its threatened closure of the strategic strait, but gave no details of the contents.

"We are in the process of studying the letter and if necessary we will respond," the official IRNA news agency quoted Ramin Mehmanparast as saying.

The head of Iran's navy, Admiral Habibollah Sayari, reiterated to Sunday's Tehran Emrouz daily that closing the strait would be easy, "like drinking water."

Iran's supreme leader, Ayatollah Ali Khamenei, has also said his country would forge on with its nuclear activities "with determination."

A new nuclear site, one that can produce 20-percent enriched uranium, has started operations in a heavily defended bunker sunk into a mountain southwest of Tehran, according to the International Atomic Energy Agency.

Senior IAEA officials are to visit Iran on January 28 to discuss suspicions over Iran's activities that were crystallised in an IAEA report two months ago.

But just before that, on January 23, EU foreign ministers are expected to announce additional sanctions on Iran targeting its oil exports and possibly also its central bank.

They would add to US sanctions signed into law last month by President Barack Obama that bar foreign companies from doing business in America if they have dealings with Iran's central bank.

A sudden slide in Iran's currency, the rial, suggested an anticipatory impact of those sanctions.

Although officials in Tehran said other factors were the cause, they have taken extraordinary steps to try to shore up the rial.

Media at the weekend reported they ordered a prohibition on currency transactions outside of banks and authorised exchange shops.

Going after Iran's oil exports is striking at the heart of the Islamic republic's economy.

Iran exports around 2.5 million barrels of oil per day, bringing in up to $100 billion last year.

Saudi Arabia, which currently exports some 10 million barrels a day, has reportedly reassured US and EU officials that it can help make up for any production gap caused by sanctions on Iranian oil.

Iran-Saudi ties are poor, especially after US allegations last October that a thwarted plot to assassinate the Saudi ambassador in Washington was hatched in Tehran.

The Saudi oil minister, Ali al-Naimi, said in Sunday's Al-Watan newspaper that "Saudi Arabia is able to produce 12.5 million barrels per day to meet the needs of the world market and satisfy any increase in demand from consumer countries."

Later on Sunday, China signed energy deals with Saudi Arabia as Prime Minister Wen Jiabao visited Riyadh.

The two countries inked several economic and cultural agreements including a Memorandum of Understanding between Saudi petrochemical giant SABIC and China's Sinopec to build a petrochemical plant in Tianjin, Saudi state news agency SPA said.