Two assailants on a motorcycle attached a magnetic bomb to the car of an Iranian university professor working at a key nuclear facility, killing him and another person Wednesday, state TV reported. The slayings suggest a widening covert effort to set back Iran's atomic program.

The attack in Tehran bore a strong resemblance to earlier killings of scientists working on the Iranian nuclear program. It is certain to amplify authorities' claims of clandestine operations by Western powers and their allies to halt Iran's nuclear advances.

The blast killed Mostafa Ahmadi Roshan, a chemistry expert and a director of the Natanz uranium enrichment facility in central Iran, state TV reported. State news agency IRNA said Roshan had "organizational links" to Iran's nuclear agency, which suggests a direct role in key aspects of the program.

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.

The U.S. and its allies are pressuring Iran to halt uranium enrichment, a key element of the nuclear program that the West suspects is aimed at producing atomic weapons. Uranium enriched to low levels can be used as nuclear fuel but at higher levels, it can be used as material for a nuclear warhead.

Iran denies it is trying to make nuclear weapons, saying its program is for peaceful purposes only and is geared toward generating electricity and producing medical radioisotopes to treat cancer patients.

Tehran has accused Israel's Mossad, the CIA and Britain's spy agency of engaging in an underground "terrorism" campaign against nuclear-related targets, including 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. All three countries have denied the Iranian accusations.

Israeli officials, however, have hinted about covert campaigns against Iran without directly admitting involvement.

On Tuesday, Israeli military chief Lt. Gen. Benny Gantz was quoted as telling a parliamentary panel that 2012 would be a "critical year" for Iran _ in part because of "things that happen to it unnaturally."

"Many bad things have been happening to Iran in the recent period," added 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."

Defiant Iranian authorities pointed the finger at archfoe Israel.

First Vice President Mohammad Reza Rahimi said Israeli agents were behind the attack, but cannot "prevent progress" in what Iran claims are peaceful nuclear efforts.

Safar Ali Baratloo, a senior security official, was quoted by Fars as also saying the attack was the work of Israelis.

"The magnetic bomb is of the same types already used to assassinate our scientists," he said.

Roshan, 32, was inside the Iranian-assembled Peugeot 405 car together with two others when the bomb exploded near Gol Nabi Street in north Tehran, Fars reported. It said the person accompanying Roshan died later of injuries at a hospital.

IRNA said an 85-year old passer-by was injured in the blast.

Fars described the explosion as a "terrorist attack" targeting Roshan, a graduate of the prestigious Sharif University of Technology in Tehran.

Roshan was a chemistry expert who was involved in building polymeric layers for gas separation, which is the use of various membranes to isolate gases. He was also deputy director of commercial affairs for the Natanz uranium enrichment plant in central Iran. According to conservative news website mashreghnews.ir, Roshan was in charge of purchasing and supplying equipment for the facility.

Natanz is the centerpiece of Iran's efforts to make its own nuclear fuel. But Iran said earlier this week it was expanding some operations to a bunker-like site south of Tehran protected under 300 feet (90 meters) of rock. The existence of the Fordo facility has been known for more than two years, but some Western officials fear the opening of the labs could be another step toward developing nuclear arms.

The conservative news website, alef.ir, posted several papers Roshan contributed. It said his specialty, polymeric layers, have uses in uranium enrichment by having uranium gas pass through filtering membranes.

Since December, Iran has held or announced a series of war games that included threats to close the Gulf's vital Strait of Hormuz _ the passageway for about one-sixth of the world's oil _ in retaliation for stronger U.S.-led sanctions.

"Assassinations, military threats and political pressures ... The enemy insists on the tactic of creating fear to stop Iran's peaceful nuclear activities," Fars quoted lawmaker Javad Jahangirzadeh as saying in reaction to the blast.

A similar bomb explosion exactly two years ago _ Jan. 12, 2010 _ killed Tehran University professor Masoud Ali Mohammadi, a senior physics professor. He was killed when a bomb-rigged motorcycle exploded near his car as he was about to leave for work.

The semiofficial Mehr news agency said that Roshan had planned to attend a memorial ceremony later Wednesday for the slain professor.

In November 2010, a pair of back-to-back bomb attacks in different parts of the capital killed another nuclear scientist and wounded one more.

The slain scientist, Majid Shahriari, was a member of the nuclear engineering faculty at Shahid Beheshti University in Tehran and cooperated with the Atomic Energy Organization of Iran. The wounded scientist, Fereidoun Abbasi, was almost immediately appointed head of Iran's atomic agency.

Shahriari's expertise _ neutron transport _ lies at the heart of nuclear chain reactions in reactors and bombs. And Abbasi, now Iran's nuclear chief, has been described as a laser expert and one of the few top Iranian specialists in nuclear isotope separation.

And in July 2011, motorcycle-riding gunmen killed Darioush Rezaeinejad, an electronics student. Other reports identified him as a scientist involved in suspected Iranian attempts to make nuclear weapons.

Rezaeinejad allegedly participated in developing high-voltage switches, a key component in setting off the explosions needed to trigger a nuclear warhead.

The United States and some allies say Iran is trying to develop nuclear weapons technology. Iran denies the allegations, saying that its program is intended for energy and medical research.

The latest blast is certain to bring fresh charges by Iran that the U.S. and allies are waging a clandestine campaign of bloodshed and sabotage in attempts to set back Iran's nuclear efforts.

"Instead of actually fighting a conventional war, Western powers and their allies appear to be relying on covert war tactics to try to delay and degrade Iran's nuclear advancement," said Theodore Karasik, a security expert at the Dubai-based Institute for Near East and Gulf Military Analysis.

He said the use of magnetic bombs bears the hallmarks of covert operations.

"It's a very common way to eliminate someone," he added. "It's clean, easy and efficient."

Meir Javedanfar, an Iranian-born analyst based in Israel, said Iran's leadership is being pushed toward a decision on whether to "retaliate or compromise" as sanctions squeeze the economy and undercut the value of the Iranian rial.

"From the international consensus that we can see against Iran, even if (Supreme Leader Ayatollah Ali Khamenei) does retaliate, it's not very likely that the pressure _ sanctions and isolation _ would ease," he said. "He's in a tight spot."

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Associated Press writer Brian Murphy in Dubai, United Arab Emirates, and Amy Teibel in Jerusalem contributed to this report.