BOGOTA (REUTERS) - Colombia's attorney general on Wednesday (Feb 8) called for a probe into whether President Juan Manuel Santos's 2014 campaign received as much as US$1 million (S$1.4 million) from a Brazilian firm, though he added that the only evidence came from an ex-senator accused of corruption.

Some of the US$4.6 million allegedly paid by engineering company Odebrecht SA to Otto Bula Bula, a former Liberal Party senator, may have gone to the Santos re-election effort, Attorney General Nestor Humberto Martinez told reporters.

"For now, the testimony of Bula is the only proof of the entrance of US$1 million into the Santos campaign. He has explained the method, time and place of the delivery of that money," said Martinez, who asked that the country's electoral commission launch an investigation.

On Tuesday night, Martinez was more emphatic about the allegations.

"It has been established that of that amount (US$4.6 million), in 2014 Mr Otto Bula sent two transfers to Colombia, which were cashed at the time, for a total sum of US$1 million, and whose final beneficiary was the campaign management of "Santos for President - 2014."

The president's campaign chief Roberto Prieto denied the accusation and Camilo Enciso, the president's transparency secretary, said the allegations were untrue. The 2014 campaign secured a second four-year term for Santos, who won the 2016 Nobel Peace prize.

Odebrecht's Colombian office declined to comment.

The company is dealing with the fallout from its role in an international bribes-for-contracts scandal unearthed by Brazilian prosecutors.

US prosecutors allege that Odebrecht paid hundreds of millions of dollars in bribes in association with projects in 12 countries, including Brazil, Argentina, Colombia, Mexico and Venezuela, between 2002 and 2016.

Martinez has said that Bula, who was arrested last month on charges of bribery and illicit enrichment, was tasked by Odebrecht with ensuring a certain number of higher-priced tolls were included in a highway construction contract.

Bula has denied the allegations.

The campaign of Santos's rival in the election, Oscar Ivan Zuluaga, is also facing an investigation by the electoral commission for receiving money from Odebrecht. Zuluaga was the candidate for former President Alvaro Uribe's right-wing Democratic Center party.