US Secretary of State Mike Pompeo on Friday urged an extension to a UN arms embargo on Iran, saying it would be "nuts" to let it expire.

Opposition from UN Security Council veto powers China and Russia is expected to block a resolution to extend the blockade beyond October.

Pompeo reiterated during a visit to Vienna that Iran should not be allowed to buy and sell weapons, calling the Islamic Republic "the world's largest state sponsor of terrorism".

"I mean that's just nuts... We're urging the whole world to join us" to extend the arms embargo, he said.

As things stand arms sanctions are set to be eased gradually from October, under a Security Council resolution blessing a 2015 deal Iran signed with world powers to limit its ability to develop a nuclear bomb -- in exchange for easing trade barriers.

The landmark deal has come under strain since the US pulled out of it in 2018.

As Washington has re-imposed crippling sanctions, Tehran has in turn stepped up its nuclear activities again since last year.

Pompeo urged Tehran to provide "full, transparent and immediate cooperation" with the International Atomic Energy Agency (IAEA), which monitors Iran's compliance with the nuclear deal.

Stepping up enrichment 

After meeting Pompeo on Friday, IAEA Director General Rafael Grossi told reporters that Iran had still not granted the agency access to two sites where it has requested access in order to clarify questions about possible undeclared nuclear activity in the early 2000s. 

But he added that he had "hope" continued dialogue would get Iran to open up.

"My objective is to get this access," Grossi said.

Citing a restricted IAEA report, Bloomberg News reported Thursday that Iran was transferring advanced centrifuges used to enrich uranium from a pilot facility into a new hall at its main Natanz nuclear fuel plant, which was hit by a fire last month.

Grossi said he would not comment on restricted reports from the agency, adding Iran had informed the IAEA of "what has been going on" in Natanz and it was part of inspectors' "ongoing work".

Austria is the latest stop on a Central European tour that has taken Pompeo to the Czech Republic and Slovenia. He will continue on to Poland on Saturday.

In Slovenia he signed a declaration that the EU member will "exclude untrusted vendors" from 5G networks, part of the Trump administration's campaign to persuade allies to exclude Chinese telecoms giant Huawei.

Austria has so far declined to exclude any vendor outright.

Friday's visit was a rare bilateral trip to Austria for a US foreign minister, although Vienna and Washington both say they value their close ties.

Austrian Chancellor Sebastian Kurz, who Pompeo will meet later Friday, visited the White House last year.

The under-construction Nord Stream 2 pipeline designed to bring Russian gas to Western Europe is a bone of contention between the two governments, with Austria's OMV one of the energy industry players involved.