The comments follow an easing of tensions between Iran and the United States, who announced a prisoner swap last month.

Last week, the International Atomic Energy Agency (IAEA) said in confidential reports seen by AFP that Iran had made "no progress" on several outstanding nuclear issues.

These include reinstalling IAEA monitoring cameras Tehran had removed from its known nuclear sites, or explaining the presence in Iran of uranium particles enriched to near weapons-grade level.

IAEA director general Raphael Grossi said on Monday he had noticed a "decrease in interest" from IAEA member states, without naming them.

"There is a certain routinisation of what is going on there (in Iran) and I am concerned about this, because the issues are as valid today as they were before," he told reporters on the first day of the IAEA board of governors' meeting in Vienna.

Diplomatic sources say the United States and the so-called E3 group -- France, Germany and the United Kingdom -- have no plan this week to censure Iran for its lack of cooperation with the IAEA.

Instead, at the behest of Washington, they will submit a joint declaration to the IAEA board meeting, which is expected to gain broad support, a source told AFP.

Last month, Iran said it has reached a prisoner exchange deal with the United States, which includes the release of five US citizens held in Tehran and several Iranians detained in the US.

"We are aware that there is a bilateral process of sorts. We have been informed by the United States about this. But when it comes to the nuclear part, (it is) not clear what is being discussed," Grossi stressed.

"There are many pressing issues on the international agenda but I think it is important to continue to support the agency in its work," he continued.

In 2015, major world powers reached a deal with Iran, under which Tehran would curb its nuclear programme in exchange for relief from crippling economic sanctions.

That started to unravel in 2018 when then US president Donald Trump unilaterally withdrew from it and reimposed sanctions. Tehran in turn stepped up its nuclear programme.

Efforts to revive the deal have been fruitless so far.

Iran has always denied any ambition to develop a nuclear weapons capability, insisting its activities are entirely peaceful.