Iran has arrested four people for trying to sabotage a nuclear site, a senior official says.
The activities of the four had been monitored for some time, said the head of Iran's Atomic Energy Organisation, quoted by local media, said
They had been caught red-handed, Ali Akbar Salehi said.
Mr Salehi did not give names or nationalities, or identify which site was the intended target.
The announcement comes amid new hopes of resolving Iran's standoff with the West over its nuclear programme.
Just over a week ago, Iran's President Hassan Rouhani spoke to US President Barack Obama by phone in a bid to ease tension over Iran's nuclear programme.
They were the first talks between Iranian and US leaders in more than three decades.
Iran's Supreme Leader Ayatollah Ali Khamenei endorsed Mr Rouhani's overtures to the US on Saturday.
"We support the diplomatic initiative of the government and attach importance to its activities in this trip," he said on his website.
Ayatollah Khamenei said that "some of what happened on the New York trip was not appropriate", without being specific.
"We are pessimistic towards the Americans and do not put any trust in them," he said.
"The American government is untrustworthy, supercilious and unreasonable, and breaks its promises."
Substantive nuclear negotiations between Iran and international powers are due to take place next week and Mr Rouhani has pledged to bring a plan to the meeting.
Iran wants the lifting of UN sanctions, which were imposed over concerns that it is seeking to build nuclear weapons clandestinely - an allegation Tehran denies.
Key Iranian nuclear sites