The Bulgarian authorities said the missile smashed into a house in the Gorna Banja district on Wednesday night.

Reports said one child was in shock, but no one was injured in the explosion.

Sofia is just 50 kilometers (30 miles) east of the Yugoslav border. President Petar Stoyanov, who rushed to the scene after the blast, urged residents not to panic.

Many Bulgarians are against the Nato strikes

"The situation of course is bad," he added. "No one expected such developments."

Nato spokesman Jamie Shea said the incident happened after a jet fighter was illuminated by a Serb air-defence radar.

"A missile was fired in self-defence in response to this threat from a surface-to-air missile," he added. "After the ground radar was turned off the missile strayed from its target and unintentionally landed in Bulgaria.''

Colonel Tomirko Tomanov, a weapons specialist with the Air Force Chief of Staff, said the missile was an "air-to-surface type, AGM 88 Harm, or a high-speed anti-radar missile which could be carried by Nato's F-16 fighters".

Three Nato missiles have already fallen in Bulgaria during the five-week air campaign against Yugoslavia, and the alliance's planes have previously violated Bulgarian airspace.

Airspace violated

The Bulgarian army said it was unable to identify a plane which violated the country's airspace at 2141 (1941GMT) on Wednesday, shortly before the missile strike.

The plane was spotted near the town of Varshets about 50km north of Sofia.

President Stoyanov urged Nato to supply Bulgaria with sophisticated radar to identify planes. He said Bulgaria would also mark its western border with lights to help pilots.

Interior Minister Bogomil Bonev was quoted saying Bulgaria would sue the pilot of the jet which fired the missile for damage caused to the house owners.

"There hasn't been such a drastic violation of our airspace so far," he added.

One resident told Reuters: "I heard the explosion and I felt horror. For Nato it is just statistics if a missile falls in one place or another but for us it means suffering."

Bulgaria split over strikes

The incident comes as Bulgaria's parliament prepares to debate a Nato request to use its airspace for raids on Yugoslavia.

The pro-Western cabinet backs the move, while the leftist opposition vigorously rejects it and has organised street protests.

The public is divided between a desire to join Nato and the European Union and sympathy for fellow Slavs and Christian Orthodox Serbs.