By Stephanie Nebehay

GENEVA, June 17 (Reuters) - For the first time, a heated race to head the main international migration agency pits four candidates vying for a post traditionally handed to an American, diplomatic sources said on Tuesday.

Director general Brunson McKinley, who is seeking an unprecedented third term at the helm of the International Organisation for Migration (IOM), faces a tough challenge from a fellow American who has official U.S. government backing.

Also standing in what diplomats describe as an open race are a former Canadian trade ambassador not officially endorsed by Ottawa and an Italian academic whom diplomatic sources say has failed to win consensus within the European Union.

Several rounds of voting are expected on Wednesday at IOM's closed-door Council, where a two-thirds majority is required.

"Ten years ago, the IOM was seen very much as a U.S.-run organisation. In the past, the Americans proposed a candidate and everybody said okay," a diplomatic source told Reuters.

"It's very interesting to have two Americans running against each other. There has not been a competitive election before -- this is a whole new world for IOM," the source added.

The IOM, set up in 1951 after the chaotic mass displacement of post-war Europe, has always been headed by an American except in the 1960s when a Dutchman served as leader.

Under McKinley, IOM director general since Oct. 1998, the 122 member-state agency has greatly expanded. It is a respected authority in heated global debates on migration which touch sensitive issues including security, jobs and trafficking.

IOM has an annual budget of nearly $1 billion and 5,400 staff help migrants in 100 countries from Sudan to Colombia.

Washington backed away from McKinley, now 65, when he decided to seek a third five-year term. The United States is among 50 member states to have ratified an amendment to IOM's constitution barring a director-general from seeking a third term, although the rule has not yet come into force.

William Lacy Swing, a former U.S. ambassador who led U.N. peacekeeping operations in the Democratic Republic of Congo, is the official U.S. government candidate. Washington says he is "the right person to lead IOM", despite his age of 73.

Luca Riccardi, who calls his native Italy "one of the crucial points of migration flows in Europe", is seen as lacking proven experience in managing a large organisation.

Sergio Marchi, a former Canadian ambassador to the World Trade Organisation with experience in migration issues, is less likely to attract some countries because he does not have official backing from his government, diplomatic sources say.

"All four have things counting for and against them, so I don't think people will be able to call this one," a diplomatic source said. (Editing by Laura MacInnis and Dominic Evans)

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