"What we agreed is something that is viable -- it is useful to bridge this gap that we are having now, it salvages the situation now," Rafael Grossi, head of the Vienna-based International Atomic Energy Agency (IAEA), told reporters after flying back from talks in Tehran.

Iran's conservative-dominated parliament passed a law in December demanding the country suspend some inspections if the US failed to lift sanctions.

The law is due to go into effect on Tuesday.

"This law exists, this law is going to be applied, which means that the Additional Protocol, much to my regret, is going to be suspended," Grossi said, referring to one of the agreements between Iran and the IAEA under which inspections take place.

"There is less access, let's face it. But still we were able to retain the necessary degree of monitoring and verification work," he said, describing the new arrangement as "a temporary technical understanding".

Grossi did not give details of precisely which activities the IAEA would no longer be able to do but confirmed that the number of inspectors in Iran would not be reduced and that snap inspections could continue under the temporary arrangement.

The new "understanding" will however be kept under constant review and can be suspended at any time.

Grossi's visit to Tehran came amid stepped-up efforts between US President Joe Biden's administration, European powers and Iran to salvage the 2015 nuclear deal that has been on the brink of collapse since Donald Trump withdrew from it.

Grossi described Sunday's agreement as "a good result... a reasonable result" following "very, very intensive consultations" with Iranian officials.

He was speaking after two days of meetings in the Iranian capital during which he met Foreign Minister Mohammad Javad Zarif and the head of the Iran Atomic Energy Organisation, Ali Akbar Salehi.

Grossi said his hope in going to Tehran was "to stabilise a situation which was very unstable".

"I think this technical understanding does it so that other political discussions at other levels can take place, and most importantly we can avoid a situation in which we would have been, in practical terms, flying blind," he added.