Up to 100 cruise missiles have been launched at military targets in Yugoslavia as the start of a NATO operation called Allied Force. The air strikes, which began at 7 p.m. Irish time, are the first launched by the alliance against a sovereign country in its 50-year-history.
A Yugoslav claim that a NATO aircraft was shot down was denied by the Pentagon in Washington.
As the first NATO air strikes hit Yugoslavia, President Clinton defended the alliance's action and said that "only firmness now can prevent greater catastrophe later."
President Clinton said early today that NATO had launched its air strikes "to defuse a powder keg in the heart of Europe".
Waves of NATO bombers from eight participating nations, including the US, Britain, France, Italy, Germany and Canada, hit targets in at least half a dozen centres in Serbia, Kosovo and Montenegro.
The Yugoslav army said early today that an unspecified number of women and children, the family of former soldiers, were killed in the air strikes. The official Tanjug news agency also reported four soldiers wounded, one seriously.
The army, in a statement carried by Tanjug, said the victims were "refugees" staying temporarily in military housing. Tanjug also reported civilian wounded in Kursumlija, southern Serbia.
The charge d'affairs at the Yugoslav embassy in Washington told CNN early today that there had been "many deaths".
In Belgrade, the initial attacks were seen as two brilliant orange lights drifting down through the city's cool, still night sky.
To the south-west of the Hyatt Hotel, a vantage point for journalists, there was a sudden eruption of orange light which shot high into the sky from behind a hill. There was no sound, just a giant cloud of pulsating orange smoke.
Seconds later, to the west, again without a sound, five magnesium flashes marked the arrival of cruise missiles at the large Belgrade air base eight miles away.
A further explosion came from the north and after a few minutes the city was ringed by fires.
People initially took no notice of the air raid sirens. Traffic kept flowing and the lights stayed on.
It was clear last night that the attacks have badly strained US relations with Russia where President Yeltsin had earlier begged President Clinton "not to take this tragic, dramatic step". The Russian leader warned: "This is war in Europe, and maybe even more."
This followed a 35-minute telephone conversation between the two leaders, which a White House spokesman described as "candid".
"President Yeltsin made it clear to President Clinton that he opposed NATO's use of force here but he also did indicate Russia's frustration with President Milosevic," the spokesman said.
Mr Clinton expressed the hope that this disagreement over the use of force would not damage their relationship.
However, after the bombing had begun, Mr Yeltsin reacted angrily and called it "outright aggression" against Yugoslavia and called for a meeting of the UN Security Council. He has withdrawn co-operation with NATO.
The UN Secretary General, Mr Kofi Annan, in a brief statement before the Security Council began meeting in New York, criticised Mr Milosevic for rejecting a political settlement.
Mr Annan said that under the UN Charter, the council "should be involved in any decision to resort to force." But there are times "when the use of force may be legitimate in the pursuit of peace."