U.S. officials, for the second straight day, took issue yesterday with press reports of major escalation of the Sino-Vietnamese war, and for the second day in a row the State Department warned the press and public against "hot stories" from the battle zone.

Yesterday's "hot" account, an Associated Press story attributed to Thai intelligence sources, said that Chinese warplanes had bombed recently arrived Soviet ammunition and materiel "deep in Vietnam" near Haiphong. Thursday's "hot story" was a United Press International account, datelined Hanoi, that the Soviet Union has begun mobilizing troops along its Chinese border. The information was attributed to "Japanese reports."

Both stories were given prominent play by some newspapers, radio and television outlets. Both stories, according to U.S. officials, appear to be completely untrue.

Since the newest Indochina war broke out last weekend, an unusually large number of erroneous, exaggerated or highly speculative news accounts have been given large amounts of space and air time in American media. Among these, according to officials, were stories that said:

The Soviet Union has put its army on alert and canceled leaves. The story was filed by Victor Louis, a Soviet journalist with reputed inside connections who writes for the London Evening News. Louis later said his information came from his son, a university student, who had picked up "village gossip" from a friend.

Chinese forces in Vietnam have begun to withdraw. This story, ascribed to Peking diplomatic sources, is said to have resulted from a "garble" by the Lebanese ambassador in the Chinese capital. He was called in for a briefing by foreign ministry officials and reportedly misunderstood the Chinese message.

Massive Vietnamese reinforcements have moved north from the Hanoi area to meet Chinese invaders. The story reportedly originated with a reporter for Japan's Asahi Shimbun who was permitted by Vietnamese authorities to observe vehicles with troops heading up a road.

Throughout history, wars have given rise to erroneous reporting, often proving the adage that "truth is the first casualty" of military conflict. But the current Sino-Vietnamese conflict lends itself to misinformation and even deliberate "disinformation" to an unusual degree.

In this border battle between two communist states, there have been no independent war correspondents on the ground with freedom to travel, observe and question. The Chinese have permitted no outsiders in the area they occupy, so far as is known. The Vietnamese have taken a number of foreign correspondents on controlled tours, but have tightly restricted their access to first-hand information.

The other nation whose actions are being closely watched, the Soviet Union, is also highly secretive about military plans and activities. Because it is a superpower with a vast military machine and about 5,000 strategic nuclear warheads, its actions are a matter of grave international importance -- but the worldwide import and fascination far outstrip the available information.

Major sources for the war reports, in these circumstances, have been communiques or propaganda broadcasts by one or another side.U.S. officials who have studied the war said that many of these accounts appear to have at least a kernel of fact, but that finding and isolating it is difficult. Vietnamese communiques, for example, are believed to locate the area of action with relative accuracy, but their account of enemy casualties -- as in the war against the American -- is considered highly inflated.

Another major source, probably the most reliable available, is the output of various national intelligence agencies. The United States, for example, listens to battlefield radio transmissions through sophisticated electronics gear, and photographs military installations and deployments with high-resolution cameras in satellites and aircraft.

Some of the information is shared with journalists and with intelligence agencies of friendly nations; other information is closely held. Some information is passed along with authorization from higher officials, often with national strategy or objectives in mind; some information is passed along out of friendship with reporters or a desire to inform the public; some information is passed covertly, at times received second- or third-hand through journalistic networks.

All of this adds up to an extremely difficult situation: a war of grave international ramifications and high journalistic interest, but reported through shadows and secondary sources of uncertain validity.

In a war aimed at political advantage and international standing more than territorial acquisition, public perceptions are at least as important as battlefield reality to the belligerents. With a wide field for jounalistic error and excess, American officials are cautioning the U.S. public to be wary.