The assassination of an Iranian nuclear scientist in Tehran Monday and the attempted murder of another suggests an intensification of clandestine efforts by the United States and Israel to sabotage Tehran's alleged drive for nuclear weapons any way they can.

The head of the Atomic Energy Organization of Iran, Ali Akbar Salehi, said the scientist who was slain, Majid Shahriari, was 'in charge of one of the great projects" at the agency.

The other man, Fereydoon Abbasi Davani, who was wounded, had been identified recently by Western intelligence as a senior scientist with the Iranian Defense Ministry.

He was also reported to have been a member of the Revolutionary Guards Corps, which controls Iran's ballistic missile forces as well as much of the nuclear program, since the 1979 Islamic revolution.

Both men were targeted by unidentified assailants on motorcycles who attached bombs to their cars in different parts of the Iranian capital while they were moving.

Second later, as the assailants sped away, the bombs exploded.

Tehran swiftly blamed the United States and Israel for the attacks, the latest in a series of covert operations against Iran's contentious nuclear program.

"Don't play with fire," Salehi warned. "The patience of the Iranian nation has its limits."

Neither the United States nor Israel, have officially confirmed that they have been engaged, with the British, on a secret war to wreck the Iranian program and to destabilize the cleric-dominated government.

But over the years there has been a chain of unexplained incidents and deaths that have beset the Iranian nuclear effort that were clearly the work of Western intelligence services or regional paramilitary forces on their payrolls.

The Americans also ran a program to lure Iranian scientists into defecting, and several reportedly have done so.

On Oct. 10, Salehi publicly acknowledged that Western intelligence services had infiltrated Iran's nuclear program.

That followed the reported arrest of several alleged spies by Iranian authorities and a mysterious attack by the Stuxnet computer worm, possibly linked to Israel, that caused havoc with computerized industrial equipment in Iran, reportedly including the Bushehr nuclear plant.

Salehi's remarkable admission, reported by the semiofficial Fars news agency, was a stark confirmation that Iran has been wrestling with a sustained intelligence assault on the nuclear program.

He said the West had intensified its drive "to establish contact with experts" at his agency and "lure them with promises of further study and better jobs abroad."

He claimed his security department had successfully countered these operations by tightening security procedures and improving privileges for nuclear scientists so they would not be tempted to defect.

Then, on Nov. 23, the International Atomic Energy Agency, a Vienna-based U.N. watchdog, reported Iran had been forced to suspect critical work on its nuclear program. That suggested the Iranians had run into serious technical problems.

The recent nuclear reverses suffered by Iran apparently have centered largely on its ability to enrich uranium, a vital process that produces weapons grade material.

The IAEA said that on Nov. 5 the Iranians were operating more then 8,000 centrifuges in 29 cascades at the Natanz uranium enrichment center in central Iran. But 11 days later, no cascades were functioning.

In July, Ivan Oelrich of the Federation of American Scientists said the Iranian centrifuges were only working at 20 percent of efficiency.

The Financial Times observed: "Iran has faced numerous technical problems with its program in recent years because of the poor quality of equipment that it is using.

"But the scale of this shutdown suggests Iran's nuclear program could have fallen victim to sabotage by intelligence agencies."

The alleged sabotage operations include explosions that wrecked the power supply at the Natanz complex in 2006. Fifty centrifuges were lost and the then-head of Iran's nuclear agency, Gholam Reza Aghazadeh, said the equipment had been "manipulated."

Intelligence sources say the CIA and its friends also plant faulty equipment in Iran's clandestine procurement pipeline. In June 2008, an Iranian businessman was sentenced to death in Tehran for supplying defective equipment to the program.

In this, the intelligence services have been aided by the fact Iran has to acquire most of its nuclear equipment abroad and use unscrupulous middle men to help it circumvent international sanctions, which makes infiltration possible.