Iran’s UN Ambassador Mohammad Khazaee appealed to Ban and the 15-nation council “to condemn, in the strongest term, these inhumane terrorist acts and to take effective steps toward elimination of terrorism in all its forms and manifestations.”

“Any kind of political and economic pressures or terrorist attacks targeting the Iranian nuclear scientists, could not prevent our nation in exercising this right” to pursue its nuclear program, Khazaee said in a letter, obtained by Reuters.

The United Nations has not heeded previous Iranian calls for UN condemnations of similar assassinations.

UN spokesman Martin Nesirky told reporters earlier on Wednesday that the United Nations was aware of the reports out of Tehran but had no immediate comment.

The United States, its European allies and Israel suspect that Iran’s nuclear program is aimed at developing atomic weapons, an allegations Tehran denies.

Several Western officials have told Reuters that Tehran may be right and the hits on Iranian nuclear technicians could be part of a plan by US or Israeli intelligence services — or both — to sabotage Iran’s nuclear program.

“Based on the existing evidence collected by the relevant Iranian security authorities, similar to previous incidents, perpetrators used the same terrorist method in assassinating Iranian nuclear scientists,” Khazaee said.

He said the assassins used “a sticky magnetic bomb to the car carrying the scientists and detonating it.”

“Furthermore, there is firm evidence that certain foreign quarters are behind such assassinations,” Khazaee said.

His letter did not explicitly blame the United States or Israel.

US officials said they had nothing to do with it.

“I want to categorically deny any United States involvement in any kind of act of violence inside Iran,” Secretary of State Hillary Rodham Clinton told reporters. “We believe there has to be an understanding between Iran, its neighbors and the international community that finds a way forward for it to end its provocative behavior, end its search for nuclear weapons and rejoin the international community and be a productive member of it.”

Earlier, State Department spokeswoman Victoria Nuland would not answer a question about whether Washington was involved in the killing — or if the administration viewed Roshan as an innocent victim. “I’m not going to speak to who may or may not have done this,” she told reporters.

The attack also came one day after Israeli military chief Lt. Gen. Benny Gantz was quoted as telling a parliamentary committee that 2012 would be critical for Iran — in part because of “things that happen to it unnaturally.”

And other Israeli officials, hinted at covert campaigns against Iran without directly admitting involvement.

“Many bad things have been happening to Iran in the recent period,” said Mickey Segal, a former director of the Israeli military’s Iranian intelligence department. “Iran is in a situation where pressure on it is mounting, and the latest assassination joins the pressure that the Iranian regime is facing.”

One former official said the magnetic-bomb attack does bear the hallmarks of an Israeli hit. Current and former US officials say Washington prefers proxies like Israel to carry out operations inside Iran, and that up until two years ago, the US and Israel coordinated actions against Iran closely. But the officials say the White House halted such cooperation after Israeli Prime Minister Benjamin Netanyahu took power.

The officials, past and present, spoke on condition of anonymity to discuss sensitive strategic negotiations.

In the event that a military intervention might be needed to halt Iran’s progress toward nuclear weapons capability, they said counterterrorist officials had considered allowing Israel to use the US-Afghan Shindand Airbase, in western Afghanistan, to launch an air strike against Iranian weapons facilities

The attack in Tehran bore a strong resemblance to earlier killings of scientists working on the Iranian nuclear program — which Iran has blamed on Israel’s Mossad, the CIA and Britain’s spy agency. They point to at least three slayings since early 2010 and the release of a malicious computer virus known at Stuxnet in 2010 that temporarily disrupted controls of some centrifuges — a key component in nuclear fuel production. But all three countries have denied the Iranian accusations.

The US and its allies are pressuring Iran to halt uranium enrichment, fearful that Iran is trying to develop nuclear weapons. Iran insists the program is for peaceful purposes only and geared toward generating electricity and producing medical radioisotopes to treat cancer patients.

Natanz is Iran’s main enrichment site, but officials claimed earlier this week that they are expanding some operations to an underground site south of Tehran with more advanced equipment.

Clinton condemned Iran in a statement Tuesday for enriching uranium at the underground Fordo bunker to a level that can be upgraded more quickly for use in a nuclear weapon than the main stockpile. She said Tehran was demonstrating a “blatant disregard for its responsibilities” and that ““there is no plausible justification” for its decision to increase enrichment to 20 percent — higher than the 3.5 percent being made at Iran’s main plant.

Speaking beside Qatar’s visiting prime minister, Clinton expanded her criticism of Iran on Wednesday and expressed concern about a series of “provocative and dangerous” threats by Iranian officials to close off the Strait of Hormuz, which connects the world to the oil-rich waters of the Persian Gulf.

“This is an international waterway,” she told reporters in Washington. “The United States and others are committed to keeping it open. It’s part of the lifeline that keeps oil and gas moving around the world.”

She said the US and its partners were making it clear to Tehran that such threats were unacceptable.

Washington and Tehran also are at odds over an Iranian court’s death sentence Monday for Amir Hekmati, a 28-year-old former US military translator who was born in Arizona and raised in Michigan. Iran says he is a CIA spy; the Obama administration flatly rejects the accusations.

It is the first time Iran has handed down a death sentence to a US citizen since the Islamic Revolution 33 years ago. Hekmati’s family says he was in Iran visiting his grandmothers.

Prime Minister Sheik Hamad Bin Jassem Bin Jabr Al Thani of Qatar, a country with deep economic ties to Iran and which exports its natural gas to the rest of the world through the Strait of Hormuz, urged more negotiations among Tehran, Washington and the rest of the international community.

“We need to find a way to live together, a peaceful way,” he said. “For us, it’s very important that we don’t trigger any military tension in the region.”