BUENOS AIRES (Reuters) - Uncertainty overshadows the Olympic football tournament after European clubs defied football's governing body by saying they were not obliged to release players.

When the action starts, Brazil hope to complete their trophy cabinet by winning the only competition to have eluded them.

The five-times world champions have attached such importance to their quest for Olympic gold that Dunga, coach of the senior national side, will take charge of their team in Beijing.

The club-versus-country row, which could end up in the Court of Arbitration for Sport (CAS), is the latest twist in soccer's uncomfortable relationship with the Games.

Titleholders Argentina have departed for Beijing despite uncertainty over whether Lionel Messi will be allowed to play.

The tournament format, which has been in use since 1992, restricts it to under-23 teams, with three overage players -- whose release is voluntary -- allowed per side.

Ronaldinho, troubled by injuries and loss of form last season with Barcelona, has been included as one of Brazil's overage players with the blessing of his new club AC Milan.

FIFA have twice in the last month told clubs they must release under-23 players for the Olympics which clashes with the qualifying stages of the Champions League in Europe.

Barcelona have taken Messi on a pre-season training camp to Scotland and said they have the backing of the European Club Association (ECA), which represents leading clubs.

ECA chairman Karl-Heinz Rummenigge said in a statement there was "no legal obligation" on clubs to release players.

Germany's Schalke 04 and Werder Bremen refused to release Brazilian pair Rafinha and Diego although both have ignored their clubs to join the national side.

Both clubs said they would take the problem to CAS which threatens to prolong the dispute indefinitely and disrupt the tournament in Beijing.

SOVIET BLOC

Football's place at the Olympics has been something of an anomaly. FIFA wants to avoid the Games overshadowing its own World Cup, which also takes place every four years.

Until 1980, the Olympic tournament was officially amateur but, in the period after World War Two, eastern European countries fielded their strongest national teams. The tournament is nowadays taken seriously in Africa and Latin America.

Brazil are in Group C where they face hosts China, Belgium and debutants New Zealand, who qualified from the easy Oceania region following Australia's move to Asia.

China, led by Chartlon Athletic midfielder Zheng Zhi, are determined to atone for being knocked out of the 2010 World Cup qualifying competition.

Argentina, who face Serbia, Ivory Coast and Australia in Group A, are strong candidates to retain the title they won for the first time in Athens.

Atletico Madrid's Sergio Aguero is set to partner Messi in an attack which has been used at senior level. Mercurial playmaker Juan Roman Riquelme, now back at Boca Juniors where he started his career, will feature as an overage player.

Nigeria and Cameroon, winners in 1996 and 2000 respectively, look dangerous although the former are set to leave behind midfielder John Obi Mikel, coach Samon Siasia having lost patience with the Chelsea player after he failed to play in the qualifying competition.

The Nigerians face United States, whose squad includes the much-hyped Freddy Adu, Netherlands and Japan in Group B.

Cameroon face Honduras, South Korea and Italy in Group D.

In the women's tournament, Brazil will attempt to win their first major tournament, inspired by number 10 Marta who has been voted FIFA's World Player of the Year twice in a row.

Brazil kick off with a tough Group F game against Germany, who beat them in last year's women's World Cup final.

Despite their recent improvement, women's football in Brazil is in a sorry state and the country does not have a national championship. United States, gold medallists in Athens, face Japan, New Zealand and Norway in Group C.

(Editing by Robert Woodward)

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