NATO said the plane, which took off from Kolberg, Poland, on the Baltic Sea, was traveling at 400 knots (450 miles an hour) when it crossed from East Germany into Western airspace, so slow as to make it unlikely it could have hostile intent. A West German Air Force spokesman said it was spotted on the radar when it was still inside Eastern Europe and was picked up definitely at Dannenberg in Lower Saxony, about 60 miles southeast of Hamburg; he said it was flying at an altitude of seven and a half miles.

Reiner Otte, the spokesman for the Second Allied Tactical Air Wing in Monchengladbach, said the pilots of the two American jets must have been surprised to see a ''MIG convertible with the top off'' - with the ejectable canopy over the pilot's seat already blown away.

According to the official account, the Soviet fighter was first spotted on the radar at 9:42 A.M. and it crashed at 10:37, apparently out of fuel, at Bellegem in Belgium, near the French border and the Channel, more than 550 miles from where it had taken off. Wim Delaere was killed in the house the MIG-23 hit while he was waiting for his parents to come back from shopping. Tass's Statement

Tass made this statement: ''Today in one of the aviation units of the Northern Troop Group a Soviet military pilot was forced to eject from his MIG-23 while carring out a training flight over Polish territory because of a malfunction of the aircraft's technical system. The pilot is alive. The plane continued its unmanned flight in a westerly direction and fell on Belgian territory. The Soviet side is in the process of contacting the states through whose airspace the plane flew.''

The plane was the second MIG to crash in Western Europe in the last month. A Soviet MIG-29 crashed June 8 at the Paris Air Show. The pilot ejected safely.