VIENNA (Reuters) - The United States warned Iran on Wednesday that it faces further international isolation and pressure if it fails to address U.N. 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 U.N. 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.

Last week, six world powers resumed talks with Iran aimed at finding a broader diplomatic settlement to a decade-old dispute that threatens to trigger a new Middle East war, but reported no substantive progress apart from an agreement to hold more talks.

(Reporting by Fredrik Dahl; Editing by Kevin Liffey)