Iran is giving its nuclear scientists more security following the assassination of scientist Mustafa Ahmadi Roshan last week, a top Iranian official said today. First Vice President Mohammad Reza Rahimi said the new security included "remarkable measures."

President Mahmoud Ahmadinejad issued orders to improve the protection of the country's nuclear scientists, Rahimi said during a memorial for the "martyrs of Iran's nuclear industry," according to Iranian news outlets. Roshan was a chemical engineer who worked at the Natanz uranium enrichment facility.

"Some remarkable measures have been taken in this regard and all people active in the nuclear sector have come under a special protection plan according to the very same order," the semi-official Fars news agency quoted Rahimi saying.

Four scientists linked to Iran's nuclear infrastructure have been killed in the past two years. Another, Fereydoon Abbasi-Davani, was targeted in late 2010 but survived and now heads the country's atomic agency.

Iran says it has identified suspects behind Roshan's death, its intelligence minister Heidar Moslehi on Sunday vowed a "crushing response" directed against the United States, United Kingdom and Israel.

Rahimi said Tuesday that Iran "has proper ways and mechanisms for giving them a response."

After Roshan was killed last Wednesday by a magnetic bomb attached to his car by motorcyclists, Iran immediately accused Israel and the U.S. of being behind the plot - and added the U.K. as an accomplice later - and said they had documents to prove Mossad and CIA involvement. American officials vehemently denied the accusation while Israeli officials remained mum. A spokesman for the Israeli Defense Forces wrote on his Facebook page that he is "certainly not shedding a tear."

Israel is widely believed to have conducted last week's assassination, or at least spearheaded it. Motorcyclists and magnetic bombs also were used in the previous attacks. Israel views Iran as the greatest threat to its security and tensions have risen in the region as speculation increases over the possibility of an Israeli military strike on Iran's nuclear facilities.

Iran has also threatened to close the Strait of Hormuz, which sees around one fifth of the world's oil production pass through it,  and Iran's military chief said American warships in the Persian Gulf would need permission to pass through. A week ago, Iran sentenced an Iranian-American to death, accusing the former Marine of spying for the CIA. Iran has naval war games planned in the Persian Gulf later this month, billed as "the greatest" ever.

It was reported Monday that the U.S. has sent a letter through multiple channels to Iran's leaders, warning that closing the Strait of Hormuz is a "red line" not to be crossed.

Also Monday, Israel's military announced that the biggest joint military exercises with the U.S. would be postponed until later this year. Foreign Minister Avigdor Lieberman said it was due to the "tensions and instability," but it remains unclear which side called them off.

According to the Tehran Times, Iran's Minister of Science, Research and Technology announced Monday that some 1,300 university students had changed majors to nuclear physics and engineering in support of the country's nuclear program.

At Sharif University, official Mehrdad Bazrpash reportedly said 300 had "announced their readiness to work in the nuclear industry promising to rob the enemies of sleep."