Hong Kong on Friday announced new targets in its bid to cut emissions from power plants, part of an ongoing effort to tackle air pollution in the Chinese city that is regularly covered in smog.

The government said it aims to slash emissions of three of the five main air pollutants measured by authorities by between 6 and 17 percent from 2017, compared to their 2015 targets announced two years ago.

The power generation sector accounts for 50 percent of the city's sulfur dioxide (SO2), 25 percent of nitrogen oxides (NOx) and 16 percent of respirable suspended particulates (RSP) emissions, according to the Environmental Protection Department.

"The tightened emission allowances will help improve the air quality in Hong Kong and the Pearl River Delta region," a department spokesman said.

"The new emission caps will require the power companies to continue their efforts to use low-emission coal as far as possible."

The new targets were announced as Hong Kong recorded "very high" pollution levels over the past week, which triggered an official health warning for those with respiratory illnesses to reduce outdoor activities.

Emissions from factories in southern China, which seep over Hong Kong's border, combine with local emissions from power plants and vehicle traffic to generate an almost daily thick blanket of haze over the teeming metropolis.

Hong Kong saw its worst pollution in years in August, which broke air pollutant readings record since 1999, except for levels reached when a natural dust storm hit the Asian financial capital two years ago.

The government announced revisions to its air quality objectives for the first time in 25 years in January, after University of Hong Kong research showed pollution-related illnesses killed more than 3,000 residents a year.