VIENNA (Reuters) - U.N. nuclear chief Yukiya Amano, a key figure in international diplomacy on Iran's disputed nuclear work, is set to win another four-year term as he faces no rivals for the post.

A letter from the chairman of the 35-nation governing board of the International Atomic Energy Agency (IAEA) to member states of the U.N. agency, dated January 7, said no other candidates had come forward by a December 31 deadline.

It means Amano, who has taken a tougher approach on the Iran nuclear file than his predecessor Mohamed ElBaradei, will almost certainly be reappointed as director general without problems, in contrast to his close election victory in 2009.

"I wish to inform you that I have received no nominations and that, consequently, Mr Amano is the only candidate," said the letter from Canadian envoy John Barrett, seen by Reuters.

Barrett indicated that he aimed for a formal decision to reappoint Amano, a veteran Japanese diplomat, at the board's next meeting in early March.

Western diplomats are generally happy with the way he has stepped up pressure on Iran to stop stonewalling an agency investigation into suspected atom bomb research, even if that push has yet to bear fruit.

But the Vienna-based U.N. watchdog's relations with Tehran, which denies Western accusations that it is seeking to develop the capability to produce nuclear weapons, have become increasingly strained since Amano took office on December 1, 2009.

Under Amano, the IAEA was criticized in 2011 for a perceived slow initial response to Japan's Fukushima nuclear disaster, but later led international efforts to agree an action plan to improve global reactor safety.

In 2009, supported largely by industrialized nations, Amano defeated South Africa's Abdul Samad Minty in a sixth round of balloting after five inconclusive votes.

(Reporting by Fredrik Dahl; Editing by Robin Pomeroy)