The Soviet Union is engaged in an "organized and methodical" campaign of executions in Afghanistan and has put to death about 300 political, religious and military leaders since invading that country, the Carter administration charged yesterday.

An estimated 900 to 1,200 Soviet troops have been killed or wounded while conducting an "extremely brutal" invasion that has resulted in a large number of civilian casualties in Afghanistan, the administration also asserted.

Reporters were provided these estimates of the toll in the Soviet invasion of Afghanistan by a senior official yesterday on condition that he be identified only as an "administration official."

The U.S. charges against the Soviets were clearly part of a stepped-up propaganda campaign timed to coincide with the U.N. General Assembly debate over a resolution calling for the withdrawal of "foreign" troops from Afghanistan.

The Soviets vetoed the resolution Wednesday in the U.N. Security Council, setting up the debate in the General Assembly, where no Soviet veto is possible.

The General Assembly debate is expected to continue today, with a vote considered likely to come Monday. A two-thirds majority is necessary to pass the resolution, which does not have the force of law.

The administration official who spoke with reporters yesterday said the United States has "reliable reports" that "an execution ground" has been established near a major Afghan city, which he declined to identify. According to the reports, about 300 Afghan leaders have been put to death there, he said.

"It's apparently taking place regularly -- a couple dozen people a day," the official said. He said the executions have been accomplished by firing squads, with "Soviet troops taking part."

The official said some of the victims appear to have been political prisoners who were not among those released Sunday from the notorious Policharki Prison in Kabul by the new Soviet-backed Afghan government.

The release of some prisoners under a widely publicized amnesty declared the administration official said.

Reports surfaced earlier this week quoting Afghan and western diplomatic sources in Kabul as saying that secret executions of political prisoners were continuing. While the Afghan government said that 2,073 prisoners were released from Policharki Prison, Kabul residents reported seeing only about 300 freed.

The official said it was difficult to obtain precise information in Afghanistan, where Soviet troops continue to clash with Afghan Moslem insurgents in the countryside. He said the estimate of 900 to 1,200 Soviet casualties was through the middle of this week, but he could not provide specific figures on numbers of Soviet dead and wounded.

"Apparently there are a large number of ambulances lined up at the Kabul airport with Russian dead to be flown back," he said.

The United States estimates there are about 85,000 Soviet troops in Afghanistan, but the official declined to characterize the extent of Soviet losses compared with overall troop strength. He said the Soviets are attempting to use native Afghan troops as much as possible in fighting the insurgents, but that they "continue to have problems with the reliability of the Afghan troops."

While no figures are available, the official said that civilian casualties "are apparently quite heavy."

"The campaign is extremely brutal," he said. "There seems to be little concern about the impact on the attitudes of civilian Afghans.

"The goal seems to be to exterminate those who oppose them. The effort seems to be relying almost entirely on brute force, and a few highly publicized instances such as the prisoner release seem to be designed as much for international consumption as internal consumption."

Asked whether the Soviets might not be undermining their objectives by their behavior in Afghanistan, the official said, "The Russians seem to have an ability to do that almost anywhere they go."

The official said the U.S. estimates of Soviet casualties come from "a variety of sources," but he would not be more specific.

The official also confirmed that a U.S. withdrawal from the Olympic Games in Moscow next summer is under consideration at the White House. Asked if the United States would like the Soviets to drop out of the 1980 Winter Olympics, scheduled for next month in Lake Placid, N.Y., the official replied, "I don't know of any plans to prevent them from coming if they wish to."