BOGOTA -- Colombia's top military leaders have agreed to cooperate in any investigation of charges that middle-ranking officers in the national Army and police are deeply involved in the persistent assassinations of hundreds of leftist militants and labor leaders.

In a report last year, Amnesty International accused the Colombian armed forces of waging a "dirty war" like the one in Argentina in which thousands of suspected leftists were killed. Asked about the charge, Defense Ministry spokesman Col. Eduardo Arevalo said, "It's probable, it's possible, that individuals in the armed forces are committing crimes, but it's not up to Amnesty International or the extreme left to judge them but to the judges of the republic."

Amnesty International said soldiers, police officers and gunmen working with them had executed, tortured and caused to disappear hundreds of left-wing activists, union leaders, students, teachers, human rights workers, lawyers who defended political prisoners and Indian leaders. Amnesty estimated the number of victims in 1986 at more than 1,000.

The Oct. 11 assassination of leftist politician Jaime Pardo Leal, an outspoken critic of the armed forces, prompted charges by leftists that the military chiefs of staff were involved. The high command issued a statement calling the charges slanderous, and no proof of a high-level plot has emerged.

"The high commanders are ready to allow any investigation, and they are ready to punish any member of the armed forces who might be involved in these crimes," Communications Minister Fernando Cepeda said in an interview.

Former attorney general Carlos Jimenez last year accused security forces of complicity in what he called an official campaign of violence. His name subsequently appeared on a death list. Since then, two of those on the list, including Pardo, have been shot down by unidentified assassins.

Newspaper columnists and the editors of news magazines now write regularly about an Argentine-style "dirty war" against leftists and critics of the military.

Communications Minister Cepeda, who is considered one of President Virgilio Barco's closest advisers, said Colombia cannot be compared to Argentina, where the military government itself directed the murder of dissidents.

"It's not the kind of dirty war that we all know in Argentina, because you can be sure that the Colombian government is not involved at all," he said.

During long meetings with government officials following Pardo's slaying, military leaders disavowed "any participation of the Army as an institution," Cepeda said. "We cannot say it is impossible that there there are individuals in that process," Cepeda added. "We are trying to clarify that."

Retired general Alvaro Valencia Tovar, a former Army commander who is considered one of Latin America's foremost counterinsurgency theorists, said he believes abuses were committed by "the isolated officers convinced that through these methods they can improve their fighting ability and the possibilities of destroying their enemy."

Colombia has been wracked by guerrilla insurgency for the last 25 years, but the principal target of recent political violence has been civilian political activists rather than armed rebels.

The Patriotic Union, or UP, has suffered the most victims. Pardo was president of the Patriotic Union and, with his death, 471 party members have been killed, according to the party's statistics. Diplomatic sources said the number is lower, but no one denies that the Patriotic Union has been hit hard.

According to government statistics, authorities are holding suspects in only three of the killings.

The Patriotic Union was launched in 1985 by the country's largest guerrilla group and the Communist Party to run candidates in last year's elections under a peace plan calling for a cease-fire and participation by the left in electoral politics. Although former guerrillas of the Revolutionary Armed Forces of Colombia, or FARC, are in the ranks of the Patriotic Union, the party's 15 congressmen and dozens of local officials have made a name for themselves as aggressive reformers rather than violent revolutionaries.

Pardo, a 46-year-old former Superior Court judge, was the Patriotic Union's presidential candidate last year. He was gradually accepted into the political establishment as a radical who played the game fairly even while trying to change the rules of Colombia's tightly controlled two-party system.

The government sent an official delegation to Pardo's funeral. He was buried among the rich and famous in the municipal cemetery.

His murder is seen as a direct assault not only on the Patriotic Union but on Colombia's democratic system, one of the longest running in Latin America. The country's elites now fear that the explosion of political assassinations will rock the political system.

A recent editorial in the Bogota newspaper El Tiempo, which speaks for Colombia's traditional elite, warned that Colombia is in "the most dangerous moment in its history."

"This has gone too far," El Tiempo editor Hernando Santos said in an interview. "You can't stand all these killings all the time. The country can't stand it."

Santos, who staunchly defends the Colombian armed forces on the editorial pages of his family's newspaper, leans toward the theory that the left killed Pardo as part of a Machiavellian move to destabilize the government, but he does not dismiss the notion that military officers carried out the crime.

"We have to accept all possibilities, including the military," said Santos. "I hope it is not true."

Colombian judges rarely have a chance to make a ruling on the frequent accusations that specific members of the armed forces are involved in murder and torture. The charges are usually thrown out for lack of evidence.

In his last press conference Sept. 1, Pardo suggested that investigating officers, who are usually officials of the Administrative Department of Security, or DAS, produce inadequate reports as a way to thwart justice. The head of DAS, which is the equivalent of the FBI, is a military officer.

In a letter sent Oct. 15 to President Barco following Pardo's murder, the Patriotic Union demanded that DAS be reorganized under civilian leadership. The next day the government asked congressional approval for a shake-up of DAS, but nothing has been said about replacing its military director.

In a televised speech Oct. 17 following Pardo's murder and two days of violent protest against the killing, Barco acknowledged the wide public skepticism of Colombia's overworked and underfunded justice system. He promised widespread reforms.

In an attempt to offset criticism that the government is doing nothing to halt the advance of violence, the Communications Ministry issued a statement Wednesday saying that 443 murder suspects had been arrested and 12 of them tried in the last year.

Prosecutors are now looking into a recent political killing that has focused public attention on the Army's intelligence service.

Following the slaying of a Patriotic Union mayor in the small, north-central town of Sabana de Torres, former soldier Gonzalo Ortega stepped forward to tell the attorney general's office that he had been hired by an Army intelligence officer to help plan the assassination.

Ortega, now under government protection, made his charges public in an interview that was published last month in the weekly news magazine Cromos.

According to Cromos, Ortega said that over the last year he had been receiving $40 a week to carry out undercover jobs for Capt. Luis Ardila, head of the military intelligence division known as S-2 in the regional capital of Bucaramanga.

Ortega told Cromos that he was sent to Sabana de Torres in August to prepare the assassination of its Patriotic Union mayor, Alvaro Garces. Ortega said Ardila believed the mayor was meeting secretly with FARC guerrillas.

After 12 days, Ortega returned to tell Ardila that he did not want to participate in killing Garces because he could find no connection between the mayor and the guerrillas, according to the Cromos interview. Ortega said he was then imprisoned and later escaped.

On Aug. 16 Garces was attacked by five assassins and murdered. Colombia has so many violent crimes, most of them nonpolitical, that newspaper editors probably would have put the story on back pages if one of the suspected killers had not been chased down, caught and found to possess a gun permit carrying the seal of the Army and what appeared to be Ardila's signature.

Ardila said the signature was false. In a letter to Cromos, he acknowledged knowing Ortega but strongly denied his accusations. But the the discovery of the gun permit and the published statement of a confessed collaborator jolted even the most fervent supporters of the military. "I was astonished," said El Tiempo editor Santos. "I called {Defense Minister Gen. Rafael} Samudio. I said: 'This guy {Ortega} is saying he received orders' " from Capt. Ardila.

Santos said he has decided that the confession published in Cromos does not carry enough detail to be convincing.

Retired Army general Joaquin Matallana is convinced that Colombia is in the midst of a dirty war and that "a consensus has been reached in all levels {of the armed forces} that to save the nation, the communists have to be eliminated."

Matallana's name appeared with 21 others on a death list published in August. Two of those on the list, Pardo and human rights activist Dr. Hector Abad, have been shot down so far. Matallana said he does not give much thought to it, but he knows he could be next.

Matallana said he thought he was singled out because he is "the only retired general who has come out openly in defense of the peace process" begun by former president Belisario Betancur.

Betancur's government negotiated a cease-fire with the FARC guerrillas that allowed them to keep their arms while launching the Patriotic Union as a political party.

The agreement was denounced by conservative military commanders and many members of the nation's traditional elite, but Matallana said the agreement simply recognized that after more than two decades of fighting, "neither the Army nor the guerrillas could defeat each other, and the best thing was to get the guerrillas into the system."

Extreme right-wing members of Colombia's landed and industrial elite have never accepted Betancur's peace process, Matallana said, and they have been holding semiclandestine meetings with military leaders to persuade them "to harden their line."

Matallana said the powerful local families who have dominated regional politics for generations feel threatened. "They have the idea that the only way to save the country is to physically eliminate all those individuals who are coming up on the left," Matallana said.