Several hundred Tanzanian troops have crossed the Kagera River into the area of Northwest Tanzania occupied by the Ugandan army for the last two weeks, Western diplomatic sources said yesterday.

But there was no independent confirmation that the Ugandans had followed Ugandan President Idi Amin's published order Tuesday to withdraw to what he described as Uganda's recognized border.

The diplomatic sources said a Tanzanian battalion had reached the northern bank of the Kagera River. The Tanzanians were sent to harass any Ugandan units still on Tanzanian soil.

In Dar Es Salaam, meanwhile, the Tanzanian government said many persons were missing behind Ugandan lines in Northwestern Tanzania and expressed fears that some were slain during the Ugandan occupation.

In the first official statement on the civilian toil in the war zone west of Lake Victoria, Tanzania said 40,000 refugees escaped "murder and carnage" by the Ugandans by heading south across the Kagera River in canoes and by any other means.

The statement said the known population of the so-called Kagera salient seized by Uganda was 50,000, leaving 10,000 unaccounted for. Their fate is being investigated by the government.

"But it is feared that a number of them may be dead or too critically wounded to move," the statement said. "Without exception, all the peasants that crossed to safety told horror stories of murder, rape, arson, sadistic brutalities and wanton torture."

The statement quoted a priest who reached safety as having reported that Ugandan soldiers burst into a church one morning, sprayed the praying congregation with bullets, and then burned the building.

The Tanzanian statement appeared aimed at providing additional justification for the government's recently adopted policy of using the war with Uganda to destroy Ugandan President Amin's government.

In another development, it was announced in Dar Es Salaam that the United States and Tanzania have signed an agreement under which Tanzanian will receive free of charge 50,000 metric tons of American corn worth more than $5 million over the next two years.