The Soviet Union has warned several Western European countries that selling arms to China is likely to cause "very big damage" to bilateral relations with Moscow and could even bring about unspecified retaliation, Soviet diplomatic sources said yesterday.

The sources, who are in a position to give an authoriative account of Moscow's policy and perspectives on the Sino-Soviet dispute, did not go into detail about either the potential damage or retaliation.

"We will not stand idle" if the sales take place, an official said.

Informed Western diplomats doubt that the Soviet Union will be able to follow through on its threats.

"In the end it will be all bluster," a Western official said.

France is reported to be in the final stages of a $350 million sale of anti-tank and antiaircraft missiles and helicopters to Peking. Britain is considering the sale of several hundred vertical-takeoff-and-landing Harrier fighter planes. The Chinese have also been shopping for arms in Italy.

Senior Soviet officials complained about these possible arms deals during Secretary of State Cyrus Vance's trip here earlier this week and voiced suspicion that the United States is fostering the sales to build up military pressure on the Soviet Union's eastern front. U.S. officials have denied the charge, saying Washington has a firm policy against arming China and is not encouraging the European to do so.

Despite the denials, "it is quite clear that appropriate circles in Washington are not against and even endorse the selling of arms to Peking by Western European countries," a Soviet official said.

The Soviets contend that military equipment sold to China may be used not only against the U.S.S.R. but against capitalist states in southern Asia and ultimately against the sellers themselves Western arms sales that strengthen China are being compared here to European arms deals with Hitler before Germany launched World War II in the 1930s.

In a display of acute sensitivity to the communist rival, Soviet officials painted this picture of activity along China's southern rim:

Chinese military pressure on Vietnam, which sought to have "a policy of equal distance" between the two communist giants but was not permitted to do so by Peking. Despite the pressure, Vietnam's well-trained and well-equiped army of a million men makes it too costly for the Chinese to take military action, in this view.

Chinese military support for the Pol Pot regime in Cambodia, which Moscow expects to be ousted in the near future, the Soviet sources said. One third of Cambodia is under the control of insurgent elements, they added. Although it is not formally acknowledged here, the insurgents are supported and perhaps even organized by the Vietnamese.

About 30,000 armed rebels in Thailand supported by China, and anti-malaysian insurgent forces in the jungles around the Thailand-Malaysia border.

Continued trouble in Burma, a "bleeding state" which is claimed to be one third under Chinese control.

Future difficulties arising from Peking in Bangladesh and ultimately, in India.

The Soviet sources tacitly conceded that Moscow is supplying military aid to Vietnam to do battle against Chinese-back Cambodia and said that military assistance programs for Southeast Asian nations are a possibility.

For the most part, though, the Soviets are saying they prefer to combat Chinese initiatives by strengthening economic and diplomatic ties in Asia.

"We don't have bases and we don't need them. We're not going to have bases in Vietnam. Our navy practically doesn't visit there. We're not going to use the fleet," a Soviet official said.

Specifically ruled out was a Soviet relationship with Taiwan, an anti-Peking idea with recurrent popularity in Asian rumor mills. However, Taiwan's representatives will be permitted to participate in the 1980 Moscow Olympics as a routine matter of international obligation, according to Soviet sources.