Glyn Davies, U.S. chief envoy to the International Atomic Energy Agency, prior to the start of the agency's 35-nation board meeting in Vienna. (Hans Punz/Associated Press)

The United States has "serious concerns" that Iran is trying to preserve a nuclear weapons option, Washington's nuclear watchdog said Wednesday in Vienna.

Ongoing uranium enrichment activity moves Iran "closer to a dangerous and destabilizing possible breakout capacity," said Glyn Davies, chief envoy to International Atomic Energy Agency (IAEA), speaking at a closed session of the agency's 35-nation board of governors.

According to a recent IAEA report, Iran already has enough enriched uranium to produce one nuclear weapon or is very close to that.

Meanwhile, Britain, France and Germany urged Iran to engage in "meaningful negotiations" to arrive at a diplomatic resolution to the international standoff over its nuclear program.

The three countries told the 35-nation board of the IAEA that Iran's refusal to be transparent and co-operative in clarifying outstanding issues "further reinforces doubts" and was "inexcusable."

Iran maintains its nuclear program is peaceful and aimed at generating electricity, though others contend it is covertly trying to build an atomic bomb.

With files from The Associated Press