Military chiefs described the assault as significantly heavier than on the first night with air raid sirens sounding constantly in Belgrade and other areas.

At one point a Serbian television channel was reported to have appealed for surgeons and for blood donors.

In Kosovo itself, unconfirmed reports have indicated renewed fighting and an alleged massacre of 20 villagers by Serb policemen.

Jon Devitt reports: "The air strikes look set to continue"

But while Nato planes returned safely to base, the military alliance's political leaders faced condemnation from several countries led by Russia, protests in several capitals and signs of possible splits within its own membership.

US President Bill Clinton made a direct address on Friday morning to the people of Yugoslavia, explaining Nato's action. He said the alliance's quarrel was not with them and that they should blame their leader, President Slobodan Milosevic, for what was happening.

Listen to President Clinton's address

In a video-taped address in English, Serbo-Croat and Russian, Mr Clinton stressed that Nato supported Kosovo remaining part of Yugoslavia and called on all Serbs to use their influence to seek an end to what he called a needless conflict.

He said the US and its allies had resorted to military action only as a last resort.

"In the end we decided that the dangers of acting are outweighed by the dangers of allowing this conflict to continue," the president said.

Cruise missiles

Nato has warned President Milosevic his armed forces will be destroyed unless he backs down over Kosovo.

More than 100 Nato planes, including US stealth bombers, F-15 and F-16 fighter jets and Sea Harriers, took part in the second night of raids.

Serb TV reports on the strikes

Four US warships also launched a barrage of Tomahawk Cruise missiles from the Adriatic Sea.

Lieutenant Commander Yvette Brownwahler, of the USS Gonzalez, called it a "major cruise attack" - far heavier than the opening volleys.

Loud blasts were heard near Belgrade and witnesses said there were at least 15 explosions around Kosovo's capital.

Among targets reported hit were:

• Barracks in Urosevac and Prizren in Kosovo
• An airport at the southern Serbian city of Nis
• A military supplies factory in Trstenik
• Military bases in Montenegro
• Golubovci airport outside Montenegro's capital Podgorica
• Explosions were heard near Danilovgrad where munitions dumps are located.

Mark Laity reports from Nato Headquarters: "All planes have returned safely"

The Serb media said two allied planes had been shot down, but Nato denied the reports.

No independent confirmation of damage or casualties in Serbia is possible as the government has ordered journalists whose countries are involved in the strikes to leave the country.

Diplomatic relations

As Operation Allied Force resumed, Yugoslavia confirmed it had broken off diplomatic relations with the United States, Britain, France and Germany.

Yugoslav Deputy Prime Minister Vuk Draskovic said the bombings meant an end to the international peace plan for Kosovo, but Belgrade was still prepared to give the province autonomy.

In a conflicting interview, he later said they would cease all operations against the ethnic Albanians if Nato stopped its bombing.

Russian outrage

Nato member Greece is opposing the action and Italy, providing air bases to launch attacks over the Adriatic Sea, caused a flurry of diplomacy from allies after suggesting negotiations should restart.

There have also been demonstrations in several countries - the most serious in Macedonia where Serbs tried to set fire to the US embassy.

Russia has forced the Security Council to vote on Friday afternoon on whether the strikes are legal.

President Boris Yeltsin described Nato's aggression as ''a gross mistake'' and has warned the US it would be held to account for the strikes.