The Zimbabwe High Court today dismissed treason charges against six of seven defendants, including the former deputy commander of the Army, dealing a blow to government claims that followers of opposition leader Joshua Nkomo planned to overthrow the government and place him in power.

Judge Hilary Squires ruled after hearing prosecution evidence in the country's first treason trial that Dumiso Dabengwa, the former head of the intelligence for Nkomo's military forces during the country's independence war, still had treason charges to answer.

The key evidence that separated Dabengwa from the other six on the treason charge is a controversial letter Dabengwa wrote in 1980 to the then chairman of the Soviet KGB secret police, Yuri Andropov, who became Soviet leader late last year.

In the letter three years ago, Dabengwa allegedly sought Soviet assistance against the then new government of Prime Minister Robert Mugabe.

All seven men still face charges of illegal possesion of weapons, a lesser offense than treason.

By dismissing the treason charges against Gen. Lookout Masuku, former deputy commander of the Army and ex-commander of Nkomo's military forces, and five other defendants, Squires in effect ruled out government charges of a conspiracy to overthrow Mugabe since only Dabengwa still faces that count.

Mugabe and several of his senior ministers have charged during the past few months that Nkomo and his Zimbabwe African People's Union have sought to oust the government using armed dissidents.

The national Army carried out a brutal offensive in January and February against the dissidents and their supporters in Matabeleland, Nkomo's tribal stronghold, reportedly killing more than 1,000 civilians.

Nkomo fled the country two weeks ago charging that the Army had been ordered to kill him. Despite government denials and guarantees of his safety, he remains in London.

The trial could have significant bearing on future relations between Mugabe's and Nkomo's parties.

Since Mugabe returned from the summit conference of the Nonaligned Movement in New Delhi 10 days ago, there has been a noticeable cooling in the rhetoric by government ministers.

In a speech yesterday to the North Korean-trained 5th Brigade, which carried out most of the offensive against the dissidents, Mugabe called for stronger military discipline and added, "Any army that turns itself into a people's enemy no longer deserves the right to defend them."

"You shoot only when the command is given and you must have internal discipline to motivate you to do the right thing always," he said.

Mugabe received another report from nongovernmental relief agencies yesterday in which it is believed they cited a death toll of more than 1,000 civilians.

Several such reports have been submitted, but the government has usually responded by saying the allegations are foreign press "propaganda."

Last weekend Josiah Chinamano, the acting ZAPU leader in Nkomo's absence, called for a merger of his party and Mugabe's Zimbabwe African National Union.

These signs of cooling of tension, however, could be fleeting in the face of a new wave of dissident violence in the southwest, which is usually a forerunner of military retaliation.

Five whites were killed and a sixth was kidnaped last week. The dissidents have threatened to kill him and six foreign tourists, who were abducted last July, unless Dabengwa, Masuku and the other five defendants were released by the end of the month.

Acquittal or a lesser verdict against the defendants could help in the release of the foreign hostages, including two Americans, if they are still alive.

Many observers assume that they are dead, despite last week's message, which was the first since their kdnaping.

Much of the prosecution case rested on claims that ZAPU's military wing had secretly brought in trainloads of arms from its wartime base in Zambia after Zimbabwe's independence and stashed the weapons on farms belonging to the party.

Squires, who presided over numerous executions of guerrillas as minister of law and order in the war-time government, said there was no evidence that the defendants had diverted the trains.

Citing a long history of hostilities between Mugabe's and Nkomo's military factions, he said he thoughtNkomo's military wing kept the weapons "for defense against possible future attack" by Mugabe's military wing..

He said there "was no direct evidence at all of any hostile intent to overthrow the government."

Squires said much of the treason case against Dabengwa would rest on exactly when the letter, dated April 28, 1980, was written. He said if it was written before April 18, 1980, the date Zimbabwe became independent, there would have been no legal government to which Dabengwa would owe allegiance.

When he took the stand later in the day to launch his defense, Dabengwa said the letter was written in mid-March but postdated to April 28, close to the date it would be delivered to Andropov by a Soviet agent based in Lusaka, Zambia.