Six world powers engaged in stalled talks with Iran over its nuclear programme said Wednesday it was "essential and urgent" Tehran cooperates with the UN atomic agency over allegations of bomb research.

The US, China, Russia, France, Britain and Germany also said they were "deeply concerned" by Iran's continued expansion of its activities despite UN Security Council resolutions calling for a suspension.

They said that after 10 failed meetings between the International Atomic Energy Agency and Iran over the past 18 months it was "essential and urgent for Iran to engage with the agency on the substance of its concerns".

The countries -- all permanent UN Security Council members except Germany -- said at a meeting of the IAEA board of governors that this included Iran fulfilling "its undertaking to grant access to Parchin," a military base near Tehran.

The IAEA believes Iran constructed a large explosives containment vessel at Parchin in 2000 to conduct experiments that it says would be "strong indicators of possible nuclear weapon development".

Iran has rejected IAEA requests to visit the site and denies wanting or ever having worked on developing a nuclear weapon. It says that the IAEA's allegations are based on faulty intelligence provided by Tehran's enemies.

The allegations on Parchin form part of a major report issued by the IAEA in November 2011 summarising information on suspected nuclear weapons research that it had been given, mostly, but not only, by foreign intelligence agencies.

IAEA head Yukiya Amano said on Monday that after the 10 meetings, the latest on May 15, with Iran on these allegations, the two sides were "going around in circles".

The IAEA's latest quarterly report on Iran, circulated on May 22, showed Iran continuing to build up its capacity to enrich uranium, which in highly purified form could be used in a nuclear weapon.

The IAEA report also detailed advances by Iran building a new reactor at Arak which could provide Tehran with plutonium -- an alternative to uranium for a bomb -- if the reactor fuel is reprocessed.

Joseph Macmanus, US ambassador to the IAEA, said that Iran has failed to provide the IAEA with detailed design information on the IR-40 reactor at Arak since 2006, calling this a "basic requirement".

Iran's ambassador, Ali Asghar Soltanieh, said however that Tehran planned to use the reactor to produce mainly medical isotopes and did not intend to reprocess the fuel in order to extract plutonium.

"We have said it again and again. The agency has recorded and reported that we do no have any reprocessing facility.... This reactor is only for producing radioisotopes," Soltanieh told reporters.

Iran's advances come in spite of numerous rounds of UN and Western sanctions aimed at cutting off Iran's access to nuclear technology and which began to cause Tehran economic problems in 2012.

The United States this week blacklisted a "major network of front companies" that serve Tehran's leaders and announced new sanctions focused on the rial currency and the auto sector.

The six powers are also involved in diplomatic efforts parallel to those of the IAEA.

The last round of such talks in April in Kazakhstan "enhanced mutual understanding of the concerns of both sides", but Iran and the six other nations "remained far apart on the substance," according to the new statement.

Soltanieh said he was "rather disappointed" by the six powers' statement in view of the "new momentum" created in Kazakhstan.

"They are planning to have another meeting.... We have to be careful not to say anything which will affect this conducive environment," he said.