With Serbia's army beginning to buckle under the weight of a three-week air offensive, NATO says Kosovo guerrillas have launched attacks on army units inside the war-torn province.
Operations by units of the ethnic Albanian Kosovo Liberation Army still holding out have been reported around the northwestern town of Pec, the western town of Djakovo, and around the north-eastern town of Podujevo.
"The UCK is still active," said NATO spokesman Mr Jamie Shea, using the group's own acronym for its organisation. "They had even managed to tie up Yugoslav forces inside Pec."
The announcement comes with the alliance desperate to find an alternative to sending its ground troops into Kosovo to halt Serbian ethnic cleansing.
Air attacks, hitting army bases and also individual tanks and guns in Kosovo itself, are starting to have effect, NATO says, with many conscripts reportedly unwilling to join up for service.
"Yugoslav forces continue to demonstrate signs of wear-and-tear as a result of our operations," Mr Shea added. "One thing we are tracing at the moment are the mobilisation problems and the manpower call-up problems the Yugoslav army is facing."
While Serb forces face problems finding volunteers, the KLA is swamped with recruits. Thousands of KLA volunteers are streaming to Albania from emigre communities across Europe, often having bought uniforms in army surplus stores.
They arrive at training camps on Albania's mountain border with Kosovo, joining hundreds more who come from the ranks of refugees recently expelled.
NATO says the Serbs have overrun all seven of the KLA's regional headquarters inside Kosovo, with the guerrillas lacking the tanks, guns and experience to fight back.
At the KLA's Geneva headquarters, the vice-president of its political wing, Mr Jaser Salihu, said: "I think the KLA is as a phoenix. A lot of Kosovo Albanians are willing to fight, but at the moment we have a lack of means."
He would not comment on reports that NATO is now co-operating informally with the KLA. But KLA units are now staging limited "hit and run" missions into Kosovo, and commanders hope that eventually NATO bombing will rob the Serbs of the tanks and guns that give them superior firepower.
The KLA's revival also brings its own dangers that Kosovo's war will spill over into neighbouring Albania.
In a fourth day of cross-border fighting yesterday, Serb shells were again reported landing around the north Albanian village of Tropoje, for many weeks a forward base for the KLA. Albania has announced that its troops will be put under NATO command, and its men may soon be joining battle with the Serbs in the mountainous frontier region.