These early Serbian planes were used for training, since they were underpowered.

The tower and the approximately 15 Serbian planes were deliberately spared.

Serbian nationalist planes flying from Udbina have crossed the border as well.

But threatening force to clear the skies of Serbian planes would cross a new threshold.

"It is a dangerous practice," a senior Administration official said of the Serbian planes' tactic.

He and American officials said that the destruction of the four Serbian planes was not part of a new get-tough policy.

Four Serbian planes were shot down in the episode.

At 6:50, a fourth Serbian plane was shot down by one of the newly arriving F-16's.

Another pilot said the Serbian planes "were immediately destroyed and became fireballs."

American F-16 fighters pursued them, warned them, and when the warnings were ignored, shot four of the Serbian planes down.