UN nuclear watchdog head Rafael Grossi is due to open talks in Iran on the eve of Tehran's deadline for US sanctions to be lifted, as US President Joe Biden called for "careful diplomacy".
The deadline, set by Iranian politicians, carries the threat of a suspension of some nuclear inspections, stoking international concern about a possible expulsion of UN inspectors.
But Iran has stressed it will not cease working with the International Atomic Energy Agency (IAEA) or expel its inspectors.
Iran and the IAEA have yet to release details on the visit by the UN body's chief Mr Grossi that runs into tomorrow.
He will "meet with senior Iranian officials to find a mutually agreeable solution, compatible with Iranian law, so that the @iaeaorg can continue essential verification activities in Iran", Mr Grossi wrote on Twitter yesterday.
"Looking forward to success - this is in everybody's interest," he added.
Iran has notified the IAEA that it will suspend "voluntary transparency measures", notably inspection visits to non-nuclear sites, including military sites suspected of nuclear-related activity, if the United States has not lifted the sweeping sanctions former president Donald Trump reimposed in 2018.
The new measures are to go into effect on Tuesday.
Iran's atomic body spokesman said last week that talks with Mr Grossi will focus on how to cease "voluntary actions beyond safeguard (measures) and how to continue cooperation".
The visit comes in the wake of Mr Biden's call on yesterday for European powers to work together to curb Iran's "destabilising" activities, a day after committing to rejoin talks on Tehran's nuclear programme.
Mr Biden told the Munich Security Conference that the US would work closely with allies in dealing with Iran after his predecessor Mr Trump took an aggressive unilateral approach.
"The threat of nuclear proliferation also continues to require careful diplomacy and cooperation among us," Mr Biden told fellow leaders via teleconference.
"That's why we have said we're prepared to reengage in negotiations with the P5+1 on Iran's nuclear program," he said, referring to the five UN Security Council permanent members and Germany.
Tehran has repeatedly said it is ready to return to its nuclear commitments on condition that Washington does so first by lifting the sanctions reimposed by Mr Trump that have dealt a heavy blow to Iran's economy.