WASHINGTON — The United States denied any role in Wednesday's killing of an Iranian nuclear scientist, the latest in a series of events that have exacerbated tensions with Iran.

The assassination of Mostafa Ahmadi Roshan was the latest in a year that has already seen new U.S. economic sanctions, threats to bar American ships from the Gulf, an Iranian death sentence to a jailed U.S. citizen and an escalation in Tehran's uranium enrichment program.

Iranian reports said two assailants on a motorcycle attached a magnetic bomb to Roshan's car of, killing him and his driver. Roshan was a chemistry expert and director of the Natanz uranium enrichment facility in central Iran, and the slaying suggested a widening covert effort to set back the Islamic republic's atomic program.

But 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 neighbours and the international community that finds a way forward for it to end its provocative behaviour, 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."

Iranian authorities blamed Israel.

One former official said the magnetic-bomb attack does bear the hallmarks of an Israeli hit. Current and former U.S. officials say Washington prefers proxies like Israel to carry out operations inside Iran, and that up until two years ago, the U.S. and Israel co-ordinated actions against Iran closely. But the officials say the White House halted such co-operation 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 U.S.-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 U.S. 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 per cent -- higher than the 3.5 per cent 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 U.S. 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 U.S. 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 U.S. 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."