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MADRID (Reuters) - Venezuelan President Nicolas Maduro is using his main election rival Henri Falcon as an “instrument” to divide the country’s opposition ahead of a May 20 vote, the head of the Organisation of American States (OAS) said on Friday.

Falcon, an ex-state governor and former ally of Maduro’s predecessor Hugo Chavez, defied an opposition boycott of the presidential election to run despite conditions critics say are rigged to ensure the ruling socialists retain control.

“We all knew that Henri Falcon would be an instrument of the Venezuelan government to divide the opposition - and his candidacy ends up proving it,” Luis Almagro, the secretary general of the 34-member OAS, told an event in Madrid.

“Henri Falcon’s candidacy benefits ‘chavismo’ (the ruling movement). We asked the Venezuelan opposition to sort the wheat from the chaff and Henri Falcon is the chaff that has separated alone,” he added in comments to Reuters at the Casa de America cultural center.

Almagro reiterated his calls for harsher sanctions against Venezuela’s “dictatorship”. He said last month sanctions should be targeted against Venezuela’s all-important oil sector to intensify financial pressure on Maduro’s government, which has presided over five years of crippling recession.

Venezuela’s main opposition coalition has rebuked Falcon for legitimizing an election they deem a farce, as Maduro’s most popular rivals are barred from standing and the election board favors the government.

Maduro denies the system is undemocratic and calls the OAS a pawn of U.S. policy.

Falcon broke with the socialists in 2010 and is now promising a “national unity” government. While most analysts give him little chance, some recent polls actually put him ahead of Maduro in voter preferences.

Falcon has said the opposition’s boycott is ineffective and will only give Maduro an uncontested win, arguing that 80 percent of Venezuelans wanted change.