A man caught on video kicking a woman down a flight of stairs at a Berlin subway station has fled to his native Bulgaria, according to media reports.

A video of him holding a bottle of beer and smoking a cigarette while violently kicking the woman in the back went viral this month (Dec) after Berlin police released it in an effort to find out the identity of the culprits.

The man is one of a group of four who follow the young woman down into the metro, with one of them kicking her as she walks down the stairs.

The woman is seen landing on her face on the floor several steps down, with other people rushing over to help her as the contents of her purse are scattered.

The gang then wander off, with one stopping briefly to retrieve a dropped beer bottle.

The woman was taken to hospital.

The incident took place in the Hermannstrasse subway station in the Neukoln area of Berlin.

Prosecutors in Berlin confirmed on Wednesday (Dec 14) that the perpetrator was a Bulgarian citizen, adding that he had left Germany and that his extradition would be demanded from the Bulgarian authorities, according to reports.

Bulgarian media on Thursday (Dec 15) named the man as a 26-year-old from Varna who allegedly has a substantial criminal record, according to the Balkan Insight website.

His family, who said they were unaware of the accident, told Nova TV that the young man and his girlfriend live and work in Germany and currently are not in Bulgaria.

According to a Daily Mail report, public prosecutors in Germany have identified the man who kicked the woman as Svetoslav Stoykov, 26, but they cannot arrest him because he has left the country.

The Mail says Stoykov is married to a woman called Tsvetanka, with whom he has three children.

A Bulgarian TV journalist who visited the family home spoke to his wife's parents through a huge iron gate at the front of the house, said the Mail.

Stoykov's mother-in-law Ibryan is reported to have said: "We don't know about the video. We haven't seen it, and I don't want to watch it."

They said that while Stoykov and his wife were in Germany, they were staying at the house and looking after the children but they refused to let the TV team inside, said the Mail.

One of the acquaintances of the attacker, a 39-year-old Bulgarian, was arrested on Monday (Dec 12), but was released a day later as no proof of his participation in the crime was found.

After the video went viral, a €2,000 (S$3,000) reward was offered by bodyguard Michael Kuehr, 54, who has worked for Lady Gaga and Charlize Theron.

Mr Kuehr was said to be so shocked by what he saw on CCTV that he wanted to help the authorities, who were struggling to find witnesses in an appeal.