Hong Kong  - Police in Hong Kong Monday stepped up their hunt for a mystery attacker who throws bottles of acid on to passing crowds from high-rise buildings.

In the latest of a series of attacks, a man in his late 40s was injured by a bottle of corrosive liquid thrown from a building in the urban Sai Wan Ho district late Sunday. He was taken to hospital but was not badly hurt.

Two days earlier, another bottle of acid was thrown from a building in the city's Shamshuipo district after dozens of shoppers were injured in similar incidents over the past 11 months.

In one single attack in June, 24 people including a 4-year-old girl were hurt when a corrosive liquid was thrown from a high-rise building in the Mongkok area.

No one has been arrested over any of the incidents which have triggered widespread fear in the high-rise city. Police believe a serial attacker or attackers may be behind the apparently random assaults.

Police commissioner Tang King-shing said the police force was "highly concerned" about the incidents and was planning to offer a fresh reward for the capture of the culprit or culprits.

Rewards totaling almost 40,000 US dollars have been offered by police following earlier attacks but have failed to lead to any arrests or significant breakthroughs.

Most people in Hong Kong live in high-rise blocks and throwing any object from a height is a specific criminal offence in the city of 7 million, one of the world's most densely-populated cities. (dpa)