NATO war planes are on standby to fly training exercises over Albania and Macedonia, which border Yugoslavia on the south

BRUSSELS, Belgium (CNN) -- Ratcheting up the pressure on the Yugoslav federation over its crackdown in Kosovo, NATO defense ministers Thursday ordered military planners to prepare for the possible use of air strikes and ground forces to intervene in the Kosovo crisis.

In addition, NATO has decided to conduct air training exercises, including simulated air attacks, over neighboring Albania and Macedonia. Those exercises could start within days.

"This exercise is going to demonstrate NATO's capability to project power rapidly in the region," U.S. Defense Secretary William Cohen said.

British Defense Minister George Robertson warned that "very serious options can be put in place at a very early date and will be," unless Yugoslav leaders end their military crackdown on ethnic Albanians in Kosovo.

CNN's Patricia Kelly reports

"[Yugoslav] President [Slobodan] Milosevic would be rash and foolish if he would ignore the message coming from NATO. The message is clear an unambiguous -- Belgrade, think again," Robertson said.

Top Yugoslav: We'll keep Kosovo 'whatever the cost'

In Belgrade Thursday, Serbian Deputy Prime Minister Vojislav Seselj said the NATO threat of military action must be taken seriously, and he warned that the Yugoslav federation would defend itself if attacked.

NATO defense ministers want both sides, including Kosovo's ethnic Albanians, to cease their military actions

"We cannot give up Kosovo, whatever the cost, and we must defend it by all means because this is the only country we have," he said.

Possible options being considered by NATO include establishing a no-fly zone over Kosovo or a heavy-weapons exclusion zone in the province; air strikes at selected military targets throughout the Yugoslav federation; a civilian air embargo; and the use of ground troops.

Kosovo is a province within Serbia, one of two republics that make up the Yugoslav federation. About 90 percent of Kosovars are ethnic Albanians, and many of them are demanding autonomy or outright independence for Kosovo, which Serbia's leaders insist they will never allow to happen.

More than 250 people have died in fighting since February, when the Yugoslav government launched a crackdown on separatists, in particular the Kosovo Liberation Army. Thousands of refugees have fled to Albania and Macedonia

NATO wants Yugoslavs to declare cease-fire

NATO countries fear the Yugoslavs are using "scorched-earth" tactics to clear villages and create a buffer zone along Serbia's border with Albania, which is believed to be the main source of weapons and support for the KLA.

Cohen

To avoid military action, the NATO allies are demanding that Milosevic declare a cease-fire in Kosovo, withdraw his forces, stop repressing the ethnic Albanians and begin a dialogue with them.

After meeting in Brussels, the NATO defense ministers issued a communiqué saying they were commissioning plans for "halting or disrupting a systematic campaign of violent repression and expulsion" against ethnic Albanians.

Cohen: U.N. support for strike 'not imperative'

Britain also has proposed a U.N. Security Council resolution authorizing the use of force to stop the Yugoslav crackdown. However, Russia, a traditional Serb ally, has so far refused to go along.

But Cohen said Thursday that it while it was desirable to have U.N. support, it was "not imperative" for NATO to do so before intervening militarily.

Asked how such NATO intervention could be justified, Cohen said, "It could, strictly speaking, be collective defense in terms of the instability that could be created by the continuation of the situation."

However, NATO officials made it clear that while they want the Yugoslav crackdown to stop, they are not endorsing independence for Kosovo, and they also want the KLA to cease military action.

"We certainly do not intend to send any signal that we are supporting the drive for independence," Cohen said.

On Thursday, Russian President Boris Yeltsin invited Milosevic to Moscow for talks on a peaceful resolution to the Kosovo crisis. The Yugoslav leader is to be in the Russian capital Monday and Tuesday, a Kremlin spokesman said.