Iran's supreme leader has accused the US and Israeli intelligence services of being behind the "abominable" assassination in Tehran this week of a nuclear scientist who was to be buried Friday.
The "cowardly murder" on Wednesday of Mostafa Ahmadi Roshan, a deputy director of Iran's main uranium enrichment plant, was committed "with the planning or support of the intelligence services of the CIA and Mossad," said Ayatollah Ali Khamenei.
In a message of condolence posted on his website, Khamenei said Iran's nuclear programme "does not depend on any one person" and "we are going to continue with determination and energy on this path."
His condemnation of the United States and Israel came amid calls in Iran's conservative press for "retaliation" against Israeli political and military officials -- but also a renewed offer for nuclear talks with world powers that collapsed a year ago to resume.
Ali Larijani, the influential speaker of Iran's parliament, said Thursday during a visit to Turkey that his country stood ready for the negotiations with the five permanent members of the UN Security Council, plus Germany.
"The negotiations can yield results if they are serious and not a game," he said, according to the official news agency IRNA.
"As long as Israel, which is considered as a cancer in the region, continues to exist, problems will never end," Larijani told a news conference after wrapping up talks with Turkish leaders in Ankara.
Iran has several times said it is willing to resume those talks, which collapsed a year ago.
But the office of EU foreign policy chief Catherine Ashton, who represents the world powers, has said it is still waiting for Tehran to formally respond to a letter she sent in October last year offering to return to the talks.
Iran is being hit by UN and Western sanctions over its nuclear programme, as well as what appears to be a covert campaign over the past two years that has resulted in the death of at least three of its nuclear scientists and the sabotage of its atomic activities.
Iran insists its nuclear programme is for exclusively peaceful purposes.
But most Western countries believe it masks a drive to develop nuclear weapons -- a suspicion strengthened though not confirmed by a November report by the International Atomic Energy Agency.
Western sanctions have been ramped up since that IAEA report came out.
The European Union is poised, in a January 23 meeting, to follow the United States in imposing extra measures designed to curb Iran's vital oil exports and payments for those exports.
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A senior US official speaking to reporters in Washington on Thursday on condition of anonymity said a new law signed by US President Barack Obama on December 31 aimed to "close down" Iran's central bank, the main clearing point for petroleum payments.
Obama has said "all options are on the table" in dealing with Iran -- including military action -- though his administration has said it is currently pursuing economic, political and diplomatic means.
The United States has strongly denied having anything to do with the Iranian scientist's murder.
Israel has largely kept silent about its suspected involvement in the attack, though a military spokesman, Brigadier General Yoav Mordechai, said on his official Facebook page that "I am definitely not shedding a tear" over Ahmadi Roshan's killing.
Israeli media also highlighted comments just before the attack by the Israeli chief of staff, Lieutenant General Benny Gantz, who said 2012 would see "things which happen to them (the Iranians) in an unnatural way."
Iran's government has written a letter demanding UN Security Council condemnation of the assassination, which it said was backed by unnamed "foreign quarters".
Ahmadi Roshan, 32, and his bodyguard/driver were killed after a man on the back of a motorbike sped up to their car, stuck in rush-hour traffic, and slapped a magnetic bomb onto it that directed a deadly blast inside the vehicle. A third occupant in the Peugeot 405 was wounded.
The attack was similar to four others in Tehran over the past two years targeting Iranian scientists. Three succeeded. In one of them though, the scientist -- who now heads the country's atomic energy organisation -- escaped just in time.
Ahmadi Roshan was to be buried on Friday, after traditional weekly Muslim prayers.
Iranian Vice President Mohammad Reza Rahimi, the foreign ministry, lawmakers and the powerful Revolutionary Guards all blamed Israel and the United States for the attack.
Russian Security Council secretary Nikolai Patrushev warned in an interview in the daily Kommersant after the killing that military escalation over Iran was likely and that "Israel is pushing the Americans towards it."
"There is a real danger of a US military strike on Iran," he said.
US-Iran tensions are already fraught following an Iranian court's death sentence this week on an American-Iranian former Marine it found guilty of spying for the CIA, and Iran's capture last month of what it said was a CIA drone.
The Guards have announced new naval manoeuvres in the Strait of Hormuz in the Gulf within the next few weeks, underlining Iran's threat to close the narrow channel -- a chokepoint for a fifth of the world's oil -- in the event of an attack or heavy sanctions.
Washington has responded by warning that closing the strait would be a "red line" Iran should not cross. It has sent a second aircraft carrier to waters just outside the Gulf, and a third was on its way.