The alliance believes that President Slobodan Milosevic is trying to mobilise reservists in Montenegro and southern Serbia for the Kosovo front.
The Serb ploy has been made as Nato prepares to deploy an enlarged military force on the Kosovo border.
That indicates that far from preparing for a withdrawal from the province, Serbian forces are preparing to bolster their border positions, perhaps to deter any forced entry by Nato ground troops.
Nato officials are unsure whether the influx of Serb troops represents an increase in the total deployment inside Kosovo, or whether the Serbian army and paramilitaries are simply compensating for casualties caused by two months of Nato bombardments.
Although there are no accurate figures, most Nato officials believe that the total deployment of Yugoslav soldiers and military police inside Kosovo has remained relatively stable at about 40,000 throughout the air campaign.
One Nato source argued: "Digging in serves two purposes because it hides them [Serb troops] from aerial attack. But it also sends a message along the lines that, `While we know that Serb forces cannot move around, we also know the points where you would have to come through to enter Kosovo, and were are going to stick there'."
The alliance believes the call-up of reservists is a risky strategy for Mr Milosevic, as it has triggered anti-war demonstrations in central and southern Serbia. Belgrade has been largely spared from the call-up, the alliance believes, because the authorities are worried about the prospect of highly visible anti-war demonstrations being held in the capital.
The French Defence Minister, Alain Richard, said yesterday that although it was impossible to determine the number of casualties in the Yugoslav military from Nato air strikes, the number was probably in the hundreds.
Meanwhile, the Spanish Prime Minister, Jose Maria Aznar, pledged his full support for the Nato air campaign in Yugoslavia, saying Belgrade must meet all the alliance's conditions before the bombing can stop. As speculation continues over Nato's unity, Mr Aznar told a news conference after meeting Nato's secretary-general, Javier Solana, that Spain and the alliance "have the same approach, the same understanding."
In a message that will be strongly welcomed by Britain, Mr Aznar said: "We agree with the alliance military strategy, fully and wholeheartedly.
"The alliance must succeed... if our policy is not successful we believe the policies of totalitarian regimes will be the winner.
"As things stand, unfortunately there is no reason, no cause for the Atlantic alliance to change its strategy or have some pause in the bombing."
Mr Aznar said there was "a certain weariness in public opinion and in political circles" but this did not concern him.
Nato's conditions for an end to air strikes are the withdrawal of Serb troops and police from Kosovo, the return of Albanian refugees, the deployment of an international peace force in Kosovo and an agreement on Kosovo's future status.Reuse content