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The Iranian authorities have arrested a "network of spies" which they say was behind the assassination of a nuclear scientist a year ago, state TV reports.

In a brief statement, the authorities said the network was linked to Israel's Mossad secret service.

Iran blamed US and Israeli agents for the killing at the time.

Separately, US Secretary of State Hillary Clinton has said international sanctions have slowed Iran's progress towards developing nuclear weapons.

Massoud Ali Mohammadi, a 50-year-old university lecturer at Tehran University, was killed by a remote-controlled bomb on 12 January 2010 as he left his home in Tehran.

Iran's intelligence ministry said Mossad had used bases in certain European and non-European countries as well as Iran's neighbouring states for the purpose of the assassination, as well as other activities, Fars news agency reported.

The ministry said months of complicated measures and access to sources in the Israeli regime led to the finding of "very important and sensitive" information about Mossad spy teams, which inflicted heavy damage on Israel's information and security structures, Fars reported.

Iran has blamed the intelligence services of Israel, the US and the UK over bomb attacks against two top nuclear scientists in November last year, Majid Shahriari, who died, and Fereydoon Abbasi Davani, another top nuclear expert who was wounded in a similar attack.

At the time, Iran's state media said Mr Mohammadi had been assassinated by counter-revolutionaries, Zionists and agents of the "global arrogance".

Washington later dismissed the accusation as "absurd".

Although described by the Iranian media as a nuclear scientist, scientists in the UK and the US said, from his substantial body of published research, Mr Mohammadi was unlikely to have been working on Iran's nuclear programme.

They said his expertise was in another field of physics altogether - quantum mechanics.

Speaking in Abu Dhabi as part of a tour of the Gulf, Mrs Clinton said sanctions had made it much more difficult for Tehran to pursue its nuclear ambitions.

She said Tehran had also been facing technical problems, but she did not get drawn into discussing a timeline.

She called for international pressure to be maintained, and urged caution about a recent Israeli government assessment that Iran was still three years away from being able to build a nuclear weapon.

It is the first time the Obama administration has so openly and publicly claimed that Iran's nuclear programme is facing difficulties, says ³ÉÈË¿ìÊÖ state department correspondent Kim Ghattas, who is travelling with Mrs Clinton.

Mrs Clinton is in the Gulf partly to urge Washington's allies in the region not to let up the pressure on the Iranian leadership and to continue enforcing UN sanctions, our correspondent adds.

There has been much controversy over Iran's nuclear activities.

Tehran says its nuclear programme is for peaceful energy purposes, but the US and other Western nations suspect it of seeking to build nuclear weapons.