Air Commodore David Wilby of the British Royal Air Force said only one launch of a surface-to-air missile has been spotted, and it wasn't successful.
But Yugoslavia's military strategy seems to be to save their best Russian-built anti-aircraft missiles for when NATO planes fly in low and slow, despite risking the loss of many of its assets in the meantime, analysts say.
NATO had expected a more vigorous air defense and, being cautious about casualties, its priority in the air campaign has been to reduce the risk that the Yugoslav anti-aircraft system will shoot down one of its planes.
NATO commanders have offered two explanations for the timidity of the Yugoslavs -- either NATO attacks have already damaged their system or they are saving it for later. "Perhaps it's an indication of the success of our operations, or a husbanding of resources," Wilby said.
Retired Maj. Gen. Edward Atkeson said,"They probably want to hold off until the attacking force is a more appropriate match for what they have."
In the first wave of attacks NATO has used cruise missiles, some heavy bombers and planes which deliver guided missiles from high altitudes.
As the campaign progresses it is likely to shift toward ground attack planes like the A-10 Warthog, which fly low and slow to pick out tanks and other small mobile targets. "These planes would be more lucrative targets for them," said Atkeson, senior fellow at the Institute of Land Warfare.
The Yugoslav tactics pose a dilemma for NATO forces that are under pressure to move as soon as possible to attack forces directly engaged in the province of Kosovo.
"It is a very sophisticated air defense system and before we put our troops in harm's way we want to concentrate on taking the air defense down then we can move on to secondary targets," said Wilby.
©1999 CBS Worldwide Corp. All rights reserved. This material may not be published, broadcast, rewritten, or redistributed. Reuters contributed to this report