The United States warned Iran on Wednesday that it faces further international isolation and pressure if it fails to address United Nations nuclear watchdog concerns about its atomic activity, which the West fears has a military purpose.

In a hard-hitting statement delivered at a board meeting of the UN International Atomic Energy Agency, U.S. envoy Joseph Macmanus accused Iran of "provocative actions", singling out the recent installation of advanced uranium enrichment centrifuges.

"We are deeply concerned with what appears to be Iran's unwavering commitment to deception, defiance, and delay," Macmanus told the IAEA's board of governors, according to a copy of his speech at the closed-door session.

Western countries fear Iran is enriching uranium to develop the capacity to build nuclear weapons and have led several rounds of international sanctions, while Iran says the program is legitimate and intended for purely peaceful purposes.

The Vienna-based IAEA has been trying for more than a year to persuade Iran to give it access to sites, documents and officials as part of a stalled investigation into suspected atom bomb research by the Islamic state.

Iran has in particular refused IAEA requests to visit the Parchin military site, where inspectors suspect explosives tests relevant for nuclear arms development took place, possibly a decade ago.

"Iran is inviting further isolation, pressure and censure from the international community, including possible additional Board of Governors action, until it meets its obligations and addresses the Board's concerns," Macmanus said.

The European Union also told the IAEA on Wednesday that Iranian stonewalling of the agency's inquiry into suspected atom bomb research was "unacceptable", and voiced deep concern about Tehran's expanding atomic work.

In a joint statement to the board, the 27-nation bloc reiterated that Iran must suspend its uranium enrichment work, something Tehran has repeatedly ruled out doing.

Diplomats said Sweden had tried to weaken the tone of the EU statement, delaying internal approval of the text, which was read out at the closed-door board session later than expected.

The revised version said the IAEA's latest report on Iran, which said Tehran was installing more advanced enrichment centrifuges, gave "cause for serious concern". It had initially said the report was "dismal reading", one envoy said.

The EU said there was a worrying lack of progress in a long-stalled IAEA investigation aimed at clarifying possible military dimensions to Iran's nuclear program.

"The EU considers ... Iran's procrastination to be unacceptable," the statement said.

Three EU states - Germany, France, Britain - are part of a group of six world powers that last week resumed talks with Iran aimed at finding a diplomatic settlement to a decade-old dispute that could trigger a new Middle East war.

The other three members of the P5+1 are the United States, China and Russia.