Western security agencies are most likely behind the killing of an Iranian scientist in an operation that underlines the complexities of the conflict over Iran's nuclear programme, analysts say.

Gunmen shot university lecturer Darioush Rezaie, 35, dead in eastern Teheran on Saturday, the third murder of a scientist since 2009.

One was killed by a car bomb detonated remotely.

Iran's responses to such incidents looked confused, but in Rezaie's case it was more muddled as the authorities spoke in strikingly different voices from the start.

London-based analyst Ghanem Nuseibeh, founder of Cornerstone Global Associates, said: "Assassinations will continue to be a tool used in this covert war. While it's impossible to tell with certainty whether Rezaie was an active nuclear scientist, his death appears to be another episode in that war.

"The Iranian narrative has been confused about Rezaie's work. This adds credence to the speculation that he has been involved in the nuclear programme."

When news of the shooting broke, semi-official news agency Mehr indicated Rezaie was involved in Iran's nuclear activities, then immediately withdrew the report.

Intelligence Minister Heydar Moslehi and officials denied Rezaie had links to the nuclear programme.

Then, when parliament speaker Ali Larijani blamed the US and Israel on state TV on Sunday, Moslehi said: "We have not found any trace of foreign spy services' involvement in Rezaie's assassination case yet."

Analysts say Iran might wish to play down accusations as the death shames its security agencies and could become a local political issue. Some said they thought US or Israeli agents killed Rezaie.

The US and Israel have said they are prepared to take military action to stop the Islamic republic becoming a nuclear power.

A US spokesman denied US involvement. Israeli Defence Minister Ehud Barak refused to comment.