Iran said on Saturday it had evidence Washington was behind the latest killing of one of its nuclear scientists, state television reported, at a time when tensions over the country's nuclear program have escalated to their highest level ever.
In the fifth attack of its kind in two years, a magnetic bomb was attached to the door of 32-year-old Mostafa Ahmadi-Roshan's car during the Wednesday morning rush-hour in the capital. His driver was also killed.
US Secretary of State Hilary Clinton denied responsibility and Israeli President Shimon Peres said Israel had no role in the attack, to the best of his knowledge.
"We have reliable documents and evidence that this terrorist act was planned, guided and supported by the CIA," the Iranian foreign ministry said in a letter handed to the Swiss ambassador in Tehran, state TV reported. The Swiss embassy represents US interests in a country where Washington has no diplomatic ties.
The spokesman for Iran's Joint Armed Forces Staff, Massoud Jazayeri, said: "Our enemies, especially America , Britain and the Zionist regime (Israel), have to be held responsible for their actions."
Iran in the past has accused Israel of causing a series of spectacular and sometimes bloody mishaps to its nuclear program. Israeli officials do not comment on any involvement in those events, although some have publicly expressed satisfaction at the setbacks.
Feeling the heat from unprecedented new sanctions, Iran's clerical establishment has brandished its sword by threatening to block the main Mid-East oil shipping route, starting to enrich uranium at an underground bunker and sentencing an Iranian-American citizen to death on spying charges.
State TV said a "letter of condemnation" had also been sent to Britain, saying the killing of Iranian nuclear scientists began after the head of Britain's MI6 spy service announced intelligence operations against states seeking nuclear weapons.
The West says Iran's nuclear program is aimed at building a bomb. Tehran says it has the right to peaceful nuclear power.
Tehran has urged the UN Security Council and Secretary-General Ban Ki-moon to condemn the latest killing.
After years of international sanctions that had little impact on Iran, US President Barack Obama signed new measures on New Year's Eve that, if fully implemented, would make it impossible for most countries to pay for Iranian oil.
Washington is requiring that countries gradually reduce their purchases of Iranian oil in order to receive temporary waivers from the sanctions.
The European Union is expected to unveil similar measures next week, and announce a gradual oil embargo among its member states, who collectively buy about a fifth of Iran's exports.
The combined measures mean Iran may fail to sell all of the 2.6 million barrels a day of exports it relies on to feed its 74 million people. Even if it finds buyers, it will have to offer steep discounts, cutting into its desperately-needed revenue.
On Tuesday shipping sources told Reuters Iran was storing an increasing supply of oil at sea - as much as 8 million barrels - and was likely to store more as it struggles to sell it.
Iran denies it is having trouble: "There has been no disruption in Iran's crude exports through the Persian Gulf ... We have not stored oil in the Gulf because of sanctions as some foreign media reported," oil official Pirouz Mousavi told the semi-official Mehr news agency on Friday.
The sanctions are causing real hardship on the streets, where prices for basic imported goods are soaring, the rial currency has plummeted and Iranians have been flocking to sell rials to buy dollars to protect their savings.
The pain comes less than two months before a parliamentary election, Iran's first since a presidential vote in 2009 that was followed by eight months of street demonstrations.
Iran's authorities successfully put down that revolt by force, but since then the "Arab Spring" has shown the vulnerability of authoritarian governments in the region to protests fueled by anger over economic difficulty.
Iran has threatened to block the Strait of Hormuz leading to the Gulf if sanctions are imposed on its oil exports, and has threatened to take unspecified action if Washington sails an aircraft carrier through the strait, an international waterway.
Military experts say Tehran can do little to fight the massive US-led fleet that guards the strait, but the threats raise the chance of a miscalculation that could lead to a military clash and a global oil crisis.
The Pentagon said on Friday that small Iranian boats had approached close to US vessels in the strait last week, although it said it did not believe there was "hostile intent."
The United States and Israel have not ruled out military action if diplomacy fails to resolve the nuclear dispute. Iran says it would retaliate if attacked.
The tension has caused spikes in global oil prices in recent weeks, although prices eased at the close of last week's trading on the prospect of reduced demand in economically stricken European countries. Brent crude fell 82 cents to settle at $110.44 a barrel on Friday.
The chances for an imminent easing of tension look even more remote as the nuclear deadlock continues because of Iran's refusal to halt the sensitive nuclear work.
Last week Iran began enriching uranium underground - the most controversial part of its nuclear program - at a bunker deep below a mountain near the Shi'ite holy city of Qom.
Nuclear talks with major powers collapsed a year ago. Iran says it wants the talks to resume, but the West says there is no point unless it is willing to discuss a halt to uranium enrichment, which can be used to make material for a bomb.