The head of the UN nuclear watchdog has said talks with Iran's top nuclear negotiator have been "positive", according to Iranian state TV.
Yukiya Amano, of the International Atomic Energy Agency (IAEA), met Iran's nuclear energy head Saeed Jalili.
"We held expanded and intensive negotiations in a good atmosphere," Mr Amano is quoted as saying.
His visit to Tehran came ahead of a meeting in Baghdad between six world powers and Iran on Wednesday.
Iran denies claims by Western nations that it is developing a nuclear weapon.
It has refused to provide the IAEA access to relevant sites, officials and documents for more than four years.
A website for one of Iran's state news channels quoted Mr Amano as saying: "Definitely, the progress of talks will have a positive impact on negotiations between Iran and P5+1."
UK, China, France, Russia, the United States and Germany - the so-called P5 + 1 - are due to take part in the Baghdad talks.
"I will not go into details but the agency has some viewpoints and Iran has its own specific viewpoints," he added.
Western diplomats quoted by Reuters news agency said Mr Amano would only visit Tehran if he believed a framework agreement to grant inspectors greater access was close.
By promising co-operation with the IAEA, Iran might be looking for leverage ahead of Wednesday's talks, they say.
Mr Jalili also sounded a positive note after the meeting.
"We had very good talks with Amano and, God willing, we will have good cooperation in the future," according to reports by Iranian state television.
However, previous negotiations have ended in failure. In February, Iran refused the IAEA's request to let inspectors visit its military site at Parchin, south of Tehran, which is believed to be involved in the country's nuclear programme.
Western powers are concerned that Iran is using its civilian nuclear programme to mask efforts to obtain nuclear weapons.
The UN, US, European Union, Canada, Japan and Australia are among those who have imposed sanctions on Iran to try to persuade the country to co-operate with the IAEA.
Those measures target areas such as the sale of oil, arms deals, financial transactions and trade in technology that could be used for uranium enrichment.