Colombia's chief prosecutor said Wednesday that suspicions of illegal campaign contributions to President Juan Manuel Santos are based on testimony of a rancher connected to the leader's opponents.

The allegations have drawn the winner of last year's Nobel Peace Prize into a widening corruption scandal rocking politicians across Latin America due to admissions of bribery by Brazilian construction giant Odebrecht.

Chief prosecutor Nestor Martinez said that the accusation, which Santos denies, has not been corroborated and is based solely on the testimony of Otto Bula, who has been jailed for allegedly channeling bribes on behalf of the Brazilian firm. Martinez, formerly a top aide to Santos, said that he alerted electoral authorities so they could investigate whether almost $1 million ended up in Santos' 2014 re-election campaign.

"Right now, Bula's sworn testimony is the only proof that $1 million entered Santos' campaign," Martinez said in a press conference in which he was hounded by journalists seeking more details about the accusation. "He's given information the way, place and time the money was delivered."

Just being associated with Odebrecht, which has admitted to paying $800 million in bribes across Latin America, is a major blow for Santos. His biggest political asset has been an internationally hailed reputation for rectitude that contrasts with the shady dealings of many of his rivals.

Santos has yet to comment, but on Wednesday his entire Cabinet issued a statement calling the accusations baseless.

"President Santos these last six years has demonstrated to Colombians that in all of his activities he is a transparent and unimpeachable leader," the statement said. "That is why it is absurd and unacceptable that people of a doubtful reputation, with a simple declaration and no proof, try now to sully his campaign."

When Odebrecht agreed in December to pay a $3.5 billion fine in the U.S. as part of a plea agreement, authorities in Colombia were the first outside Brazil to arrest former officials accused of taking bribes.

"So far no official from my government has been accused of taking bribes from Odebrecht, but if that should occur I want the entire weight of the law to fall on them," Santos said last month.

Among those jailed was Bula, a little-known rancher who was a regional political ally of the senator cousin of former President Alvaro Uribe, Santos' chief opponent. According to the plea agreement, officials in Uribe's government received the bulk of $11 million that Odebrecht admitted to paying in bribes in Colombia between 2009 and 2014.

According to Martinez, Bula lobbied on behalf of Odebrecht and helped channel $4.6 million to still unknown recipients after the company was awarded a major highway contract. Most of the money went through companies in Panama and China, but two transfers to Colombia of almost $1 million total purportedly ended up in the management of Santos' campaign, Martinez said.

Santos' aides quickly repeat their assertion that Santos took no private contributions, from individuals or companies, during his 2014 campaign in which he narrowly defeated Uribe's former finance minister.

Transparency Secretary Camilo Encisco said Santos welcomed an investigation to remove any doubt about his probity.

"It's the word of a criminal, who is looking for legal benefits at any cost, against the word of a campaign manager," Enciso said. "We're certain that these investigations will reveal such affirmations to be false as has occurred on previous occasions."


Associated Press writer Joshua Goodman in Caracas, Venezuela, contributed to this report.