Invading Chinese forces have stopped their advance into Vietnam after penetrating approximately six miles; and some Chinese troops may be pulling back, Western intelligence sources said yesterday.

However, American analysts said yesterday they noted no withdrawal of chinese soldiers or slackening of the assault. In an interview with The New York Times, a Chinese official said the assault would continue.

Vietnamese officials said their troops inflicted heavy casualties on the Chinese, killing 3500 men and destroying more than 100 tanks since the invasion started on Saturday.

Hanoi radio reported that Chinese planes have bombed factories, power plants and communications facilities i North Vietnam, inflicting "terrible damage."

The heaviest fighting is taking place at the northeast and northwest border areas, according to Hanoi radio.

A Soviet statement yesterday warned China to stop its invasion of Vietnam "before it is too late" and said the Soviet Union "will honor its obligations" under its treaty with the Vietnamese, which provides for "consultation" in the event that one of the signatories comes under attack.

According to unconfirmed reports, the Soviet Union has placed its troops on alert.