BOGOTA, Colombia (Reuters) - Human Rights Watch sharply criticized Colombian President Alvaro Uribe for what it called a "wildly improper response" to charges that his government's intelligence service helped paramilitaries kill civilians.

The Columbian press has reported that the Administrative Security Department, or DAS, cooperated with the far-right militias, prompting the famously short-tempered Uribe to accuse reporters of maliciously harming the country's democratic institutions.

"Instead of attacking the news media for reporting allegations of criminal activity, President Uribe should ensure a full investigation of the charges," New York-based Human Rights Watch said in a statement late on Sunday.

Uribe singled out journalists and commentators including Ramiro Bejarano, a lawyer who served on a commission appointed last year to investigate the DAS.

"Our investigation showed the paramilitaries had deeply infiltrated the DAS," Bejarano told Reuters.

"This is as serious as if, in the United States, the FBI had been infiltrated by the Mafia," he said. "Uribe's reaction has been not to clean up the agency but to attack journalists covering the story."

A government spokesman disagreed, saying it is taking steps to ensure the DAS operates legally.

Magazines Cambio and Semana published interviews with a disgraced ex-member of the DAS who accused it of protecting paramilitaries while helping some militia commanders target labor leaders for assassination.

Semana also quoted the source saying the paramilitaries helped Uribe get elected by using fraud in 2002.

Uribe dismissed Semana as "frivolous" and portrayed its editor as a high society fop.

"President Uribe's aggressive response raises suspicion about whether he actually wants the truth known, and it has a chilling effect on freedom of expression," Human Rights Watch said.

Uribe, popular for cutting crime as part of his crackdown on left-wing rebels fighting a four-decade-old insurgency, is expected to win re-election on May 28 despite the DAS probe.

"There is a big teflon factor for Uribe," said Eduardo Gamarra, political analyst at Florida International University. "It does not matter what accusations come up. Chances of him winning a first round victory remain high."

Opposition politicians say Uribe has given soft treatment to paramilitaries who have agreed to lay down their arms in return for reduced prison sentences for crimes including massacres and torture.

Critics of the demobilization say the militias, organized by landowners in the 1980s to fight off the rebels, are not being forced to dismantle their cocaine-smuggling networks as part of the deal.