VIENNA (Reuters) - Myanmar agreed on Tuesday to grant the U.N. nuclear watchdog wider inspection powers in the previously army-ruled Asian state, a move expected to help ease any lingering concern about its nuclear ambitions.

Myanmar's foreign minister, Wunna Maung Lwin, signed the so-called Additional Protocol in a brief ceremony with Director General Yukiya Amano of the International Atomic Energy Agency on the sidelines of the IAEA's annual member state meeting.

Once ratified by Myanmar, it will allow the Vienna-based U.N. nuclear agency to carry out snap inspections outside of declared nuclear facilities, a vital tool at its disposal to detect any non-peaceful nuclear activities in countries.

Myanmar has denied allegations made by an exile group three years ago that it was trying to develop nuclear weapons, and most experts say its technological expertise is still far short of that level.

But in early 2011, diplomatic sources in Vienna said the IAEA had written to Myanmar seeking information about its activities, suggesting it wanted to send inspectors there.

Lwin said in a speech to the IAEA conference that the mere existence of nuclear weapons in the world "poses a threat to the security of all nations".

Western countries have lifted or suspended sanctions imposed during nearly half a century of repressive military rule in Myanmar. But human rights activists and ethnic minority groups remain wary about the government of President Thein Sein, a former general now heading a quasi-civilian government.

In 2010, a U.N. report suggested that North Korea, which has left the nuclear Non-Proliferation treaty (NPT) and tested three nuclear devices, might have supplied Myanmar as well as Iran and Syria with banned atomic technology.

A Norwegian-based exile group said the same year that Myanmar had a secret program to develop the means to make nuclear arms. Myanmar is a member of both the NPT and the IAEA.

A U.S. think-tank said Myanmar's signing of the Additional Protocol was a promising step to address international concerns about "rumored nuclear activities, suspicious procurements, and cooperation with North Korea on nuclear proliferation".

Myanmar should end any remaining military cooperation with North Korea, the Institute for Science and International Security (ISIS) added. It should also "grant the IAEA access to any information it requires to clear its nuclear case," it said.

(Reporting by Fredrik Dahl; editing by David Evans)