Iran on Friday buried a top scientist it said was killed in an Israeli-American covert campaign against its nuclear programme, as a US-led drive for crippling sanctions ran into opposition even from allies.
Diplomats in Vienna, meanwhile, said the UN nuclear watchdog would send its chief inspector to Iran at the end of the month.
Mostafa Ahmadi Roshan, a deputy director of Iran's main uranium enrichment plant, was given a funeral service in north Tehran after noon prayers, state media reported.
He and his driver were killed on Wednesday when two men on a motorbike slapped a magnetic bomb on his car.
Supreme leader Ayatollah Ali Khamenei said the "abominable" and "cowardly" killing was committed "with the planning or support of the intelligence services of the CIA and Mossad" of the United States and Israel.
He said in a statement that Tehran would "continue with determination" its nuclear activities, which Western governments suspect mask a drive for a weapons capability despite repeated denials.
Some media close to Iran's conservatives have called for "retaliation" against Israeli officials. Tehran has demanded that the UN Security Council condemn the "terrorist" killing.
Washington has strongly denied any involvement with the assassination, although Defence Secretary Leon Panetta admitted: "We have some ideas as to who might be involved."
The prime suspect is widely seen as Israel, as it was in the murders of three other Iranian scientists in similar circumstances over the past two years. Israel has a policy of not commenting on intelligence matters.
In separate letters addressed to the US and British governments, Iranian Foreign Minister Ali Akbar Salehi protested at what he called their support for "terrorist" actions.
Washington has been leading a campaign to bring Iran's economy to its knees by slapping unilateral Western sanctions on its vital oil exports.
President Barack Obama last month signed a law targeting Iran's central bank, which clears most of the oil payments, and US envoys have fanned out to convince other nations to come on board or risk being barred from doing business in America.
The aim is to "close down" Iran's central bank, a senior US official said.
Companies from China, Singapore and the United Arab Emirates have already been hit for failing to meet the terms of the new American sanctions, the US Treasury Department said on Thursday.
EU foreign ministers are to meet on January 23 to consider new sanctions.
But Iran's two main allies on the world stage, Russia and China, have strongly criticised the new Western measures and remain adamantly opposed to any additional UN sanctions.
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"Additional sanctions against Iran, as well as potentially any military strikes against it, will unquestionably be perceived by the international community as an attempt at changing the regime in Iran," Russian Deputy Foreign Minister Gennady Gatilov said on Friday.
"This line of action undermines the international community's efforts at resolving the Iranian nuclear problem," he was quoted as saying by the Interfax news agency.
Even some US allies -- those which rely significantly on Iranian oil -- are proving reluctant.
"The United States would like to impose sanctions. We believe it is necessary to be extremely circumspect about this matter," Japanese Foreign Minister Koichiro Gemba said.
Iran has met the covert killings and the sanctions push with defiance, starting production at a new plant able to enrich uranium closer to the threshold that could be used in nuclear bombs.
It has also warned that if attacked or strangled by sanctions, it could close the Strait of Hormuz at the entrance to the Gulf -- a chokepoint for 20 percent of the world's sea-borne oil transport.
British Prime Minister David Cameron, on a visit to Saudi Arabia, warned on Friday that the "whole world" would take action if Iran closed the strait, in an interview with the Al-Arabiya news channel.
"It is in the interests of the whole world that those Straits are open and, if there was any threat to close them, I am sure the whole world would come together and make sure they stayed open," he said.
The United States has warned shutting Hormuz would be a "red line" that Iran should not cross, and has sent two aircraft carriers to waters near the Gulf to replace one that Iran's military had warned away as it wound down its deployment.
Dmitry Rogozin, Russia's departing ambassador to NATO, called for calm amid the sabre-rattling.
"Iran is our neighbour... If Iran is involved in any military action, it's a direct threat to our security," he told reporters in Brussels. "Once again, you need to have some cold mineral water and calm down."
Amid the climbing tensions, diplomats in Vienna said the International Atomic Energy Agency was to send its chief inspector and deputy director to Iran for a week on January 28.
In November, the IAEA issued its most damning report to date on Iran's nuclear programme, strongly suggesting it had military dimensions.
It was not immediately clear whether the IAEA delegation would visit Iranian facilities or just meet officials, one diplomat said.
Ali Ashgar Soltanieh, Iran's ambassador to the agency, told AFP details of the visit were still being worked out and that he hoped they would be "finalised" early next week.
And a defiant Iranian President Mahmoud Ahmadinejad, heading home after a tour of four Latin American allies, vowed to resist pressure on Tehran's nuclear programme.
"They insult our country and our people. It's clear that the Iranian people will resist," he told reporters in Quito.