Tehran today accused the west and Israel of dispatching a hit squad against its atomic programme, after an Iranian nuclear scientist was killed and another injured in co-ordinated attacks.

The attackers rode up on motorcycles and stuck bombs to the windows of the scientists' cars as they were leaving their homes in Tehran on the way to work. Seconds later the bombs detonated.

One bomb killed Majid Shahriar, of the nuclear engineering faculty at the Shahid Beheshti University in Tehran. His wife was in the car with him and was wounded. The second bomb injured Fereidoun Abbasi, 52, a nuclear physicist and professor at Shahid Besheshti, and also injured his wife.

Both men were senior figures in nuclear research. Abbasi, a former Revolutionary Guard, is named in a UN Security Council resolution imposing sanctions on Iran as working in banned nuclear activities with Mohsen Fakhrizadeh, the scientist accused by western governments of running a secret nuclear weapons programme.

Shahriari had no known links to banned nuclear work, but was highly regarded in his field. He co-authored an academic paper on fission in nuclear reactors with Ali Akbar Salehi, the head of Iran's Atomic Energy Organisation. An Iranian news website said he was designing a "new generation of theoretical nuclear reactors".

The attacks were similar to the assassination in January of Masoud Ali Mohammadi, an expert on particle physics, killed by a remote-control bomb strapped to a motorcycle as he was leaving his Tehran home on his way to work.

The Iranian president, Mahmoud Ahmadinejad, blamed "western governments and the Zionist regime" for the attacks.

Both the US and Israel are reported to be conducting covert operations aimed at slowing Iran's nuclear programme, which Tehran insists is for peaceful purposes but which western governments say is a cover for developing a nuclear warhead. Earlier this year a computer worm, Stuxnet, hit computers around the world but appeared to affect Iranian industrial plant disproportionately, particularly in the nuclear programme. Today, for the first time, Ahmadinejad admitted the worm had affected Iran's uranium enrichment. "They succeeded in creating problems for a limited number of our centrifuges with the software they had installed in electronic parts," the president said. "They did a bad thing. Fortunately our experts discovered that, and today they are not able [to do that] anymore."

Last week the UN nuclear watchdog, the International Atomic Energy Agency, reported that the main centrifuge plant at Natanz stopped enriching uranium altogether on November 16, and that there had been a steady decline in functioning centrifuges over the year. Nuclear experts said it was unclear what caused the problems, and speculated it could be a mix of design faults and sabotage.

Other possible suspects include a Sunni rebel group, Jundullah, which today broadcast a video confession of a man it described as a nuclear worker, who admitted the existence of an Iranian weapons programme. There was no immediate way of authenticating the video, and Jundullah made no claims of responsibility for today's bomb attacks.

The People's Mujahedin group has also been blamed by the Iranian government for terrorist incidents in the past, but has claimed in recent years to be pursuing a non-violent path for change in Iran.

Opposition Iranian bloggers were today debating the possibility that the killings were the work of the Revolutionary Guards or the state security services, with the aim of punishing the giving-away of secrets, or of preventing defections. However, there was no evidence to justify any such theory, or explain why Tehran would kill valued academics in this manner, rather than detain them or move them to the guarded compounds on which many Iranian nuclear specialists work, minded by bodyguards.

The attacks have come at a tense time, as the international community ramps up economic sanctions against Tehran, and with the possibility of an Israeli military attack hanging over the nuclear programme. US diplomatic cables leaked this week revealed that King Abdullah of Saudi Arabia had repeatedly urged the United States to mount such a military attack.

Iranian officials are due to meet with diplomats from six major powers on Sunday, but there has not yet been agreement on whether the meeting should take place in Turkey (as Tehran wants) or in Switzerland. The meeting would discuss Iran's nuclear programme and a range of other regional security and economic issues.

Who targeted the scientists?

Israel The country is pursuing covert operations aimed at hobbling Iran's nuclear programme, as a less costly alternative to mounting a full-scale military attack. Israeli officials admit that such operations are in progress but they give no detail. Israel is widely believed to be responsible for the Stuxnet computer worm that caused problems for the Iranian uranium enrichment programme. The country also has a record of using assassination as a weapon in what it regards as vital national interests.

United States George Bush, when he was president, ordered a covert programme to sabotage Iran's nuclear ambitions, and that mission is still believed to be under way. Targeted killings have been approved against terrorist targets, but the revelation that a US-backed hit squad was killing civilian scientists would be immensely damaging to the administration.

Jundullah The "Soldiers of God", a Iranian Sunni group based in Iranian Baluchistan, has carried out a series of bloody attacks in recent years that have largely been focused on military targets. The group, known as the People's Resistance Movement of Iran, broadcast a video yesterday of a captive who it claims is a worker from a secret nuclear facility. It did not claim responsibility for the morning's attacks on Iranian nuclear scientists.

The People's Mujahedin (MeK or PMOI) This longstanding rebel group carried out several bombings and assassinations in the years following the 1979 revolution, but has more recently presented itself as a non-violent group. It is focusing its efforts on having itself removed from the US state department's list of terrorist organisations.