The Clinton administration says NATO has approved contingency plans to use firepower against advancing Serb forces in Kosovo and that a "humanitarian catastrophe" could envelop tens of thousands of ethnic Albanians who have fled their homes.

The plans, which are centered on an aerial assault, are in the process of undergoing final "refinements," State Department spokesman, James P. Rubin, said Monday. He called the development important.A NATO official, speaking from the organization's headquarters in Brussels, Belgium, said that "by next week all the planning will be more or less finished."

The NATO Council sought to demonstrate its resolve to Yugoslav President Slobodan Milosevic by postponing its usual August vacation, State Department officials said.

There was no immediate word on what would trigger an attack by NATO. Nor was it clear whether the administration was trying mostly to unnerve Milosevic by raising the threat again publicly.

In Kosovo, a weekend of fighting displaced tens of thousands of people, some of them forced to take refuge in forests outside towns and beyond the immediate reach of relief organizations.

"The living conditions are clearly deplorable," Rubin said. "What I'm talking about is the humanitarian catastrophe that could occur in a matter of weeks if we don't get the aid to the people who are in desperate need."