Skopje (AFP) - Fierce storms packing strong winds and torrential rains overnight killed at least 21 people in Macedonia's capital of Skopje, the health minister said Sunday.

The freak weather included winds blowing at more than 70 kilometres (43 miles) an hour and resulted in flash floods and landslides, with cars swept away by the violent torrents.

Meteorological officials said the average rainfall for the whole of August fell on Skopje in the space of just two hours.

"Twenty-one people died in the areas of Singelic, Stajkovci, Smilkovci and on the ring road," the interior ministry said in a statement, referring to villages on the city's outskirts.

At least 56 people were injured in the storms, while a hospital source told AFP that an eight-year-old girl was among the dead.

"This is a disaster, we have never experienced such a thing," said Skopje's Mayor Koce Trajanovski, adding later that a "water bomb" had struck the city.

- 'Water ran so fast' -

Some of the victims died in their cars as they were rapidly engulfed in mud and water while others were unable to flee their homes in time to reach safety.

A truck driver who managed to escape from his vehicle said he was unable to help a couple with a baby in a car that was washed away before his eyes.

"The water was so fast... I do not know what happened to them," he told Kanal 5 television channel.

Local residents with bags of belongings waded knee-deep through the floods on Sunday, as damaged and overturned cars and other debris lay strewn in the mud.

An emergency was declared in Skopje and certain parts of the northwestern city of Tetovo, where heavy storms caused damage to property but no casualties.

Rain began falling at 5:30 pm (1530 GMT) on Saturday and stopped only around 9:30 on Sunday morning, with the peak of the storm in the middle of the night, around 3:30 am.

Reports said the water level reached as high as 1.5 metres (five feet) in some of the affected areas, which were being combed by Macedonia's police and army for survivors and other victims.

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"Everything was a mess. Televisions, the fridge, the sofa, everything was floating... it was a nightmare," said Baze Spriovski, a 43-year-old in Singelic, who remained without electricity.

- 'Really horrific' -

Macedonia's weather service said 93 litres of water per square metre fell in two hours on Skopje -- equivalent to the average for an entire month of August.

Meteorologists said more than 800 lightning strikes were recorded in the first two hours of the storm.

"There were thunderbolts with lightning almost every second. It was really horrific," said Biljana Joneska, 62, in Skopje.

Municipal authorities urged people to avoid going out in the streets, and especially to heep out of their vehicles, with several roads in the capital still flooded and traffic interrupted on the city's ring road.

"We are searching every metre and our activities will continue over the next few days," said Interior Minister Mitko Cavkov.

Severe thunderstorms also affected the holiday spot of Lake Ohrid in the southwest.

Skopje previously suffered disastrous flooding in 1962, a year before a huge earthquake that almost destroyed the city.

In the spring of 2014, the Balkans region was hit by its worst floods in more than a century, which affected 1.6 million people and left 47 people dead in Serbia and Bosnia.