Iran has allowed a foreign leader to visit its main uranium enrichment site for the first time.
Tsakhia Elbegdorj, the president of Mongolia, toured the Natanz facility late on Monday.
It will be seen as an attempt by Tehran to show the West that its policy of diplomatic isolation has failed.
State media said the Mongolian leader was free to visit any part of the site.
Natanz, in central Iran, has been visited regularly by UN inspectors, as well as foreign diplomats and reporters.
Iran rejects Western claims that the aim of its nuclear programme is to develop atomic weapons.
The Iranian government says it will be used for peaceful purposes, such as generating power and creating new medical treatments.