BEIJING - Several provincial-level regions on Monday started implementing the "China VI" vehicle emission standards ahead of schedule to ramp up efforts against a major source of air pollution.

Sales and registrations of new vehicles in regions including Beijing, Shanghai, Tianjin, Hebei province and Guangdong province now have to comply with what is believed to be one of the world's strictest rules on automobile pollutants.

In Beijing, all new buses and other heavy-duty diesel vehicles shall follow the new emission rules, while all new vehicles are expected to follow suit starting Jan. 1, 2020.

All existing vehicles on the roads are obliged to meet the previous "China V" emission standards.

According to official data, emissions from some 6.2 million vehicles were responsible for 45 percent of Beijing's concentration of small, breathable particles known as PM2.5, a key indicator of air pollution.

Compared with the "National V" standards, the new rules demand substantially fewer pollutants such as nitrogen oxides and particulate matters and introduce limits on particulate number and ammonia.

The new emission standards were initially set to take effect nationwide from July 1, 2020. A three-year action plan on air pollution control released last July urged early implementation in major heavily-polluted areas, the Pearl River Delta region, Sichuan province and Chongqing municipality.

Automakers and the market have been preparing for the tougher rules.

Manufacturers have completed the development of most "China VI" models and have entered the stage of mass production and sales, said Liu Youbin, a spokesperson with the Ministry of Ecology and Environment.

By June 20, 99 light vehicle makers had unveiled environmental protection information of 2,144 new models and 60 heavy-duty vehicle manufacturers unveiled information on 896 green models, Liu said.

"The market has basically accomplished a smooth transition," Liu said.

Li Hong, an official with the China Association of Automobile Manufacturers (CAAM), said roll-outs of "China VI" vehicles as well as preferential tax and fee policies would boost China's auto market.

"The production and sales of new energy vehicles (NEVs) will continue its relatively fast growth," Li said.

Car sales in China continued to drop in May, with about 1.913 million vehicles sold, down by 16.4 percent year on year, CAAM showed. Bucking the trend, sales of NEVs kept growing that month, edging up 1.8 percent year on year.

China saw robust sales growth of NEVs in the first four months this year with 360,000 NEVs sold, surging by 59.8 percent from the same period a year earlier.

Chinese authorities have announced that the tax exemptions on NEV purchases will continue through 2020 to boost the country's green development and retain a strong domestic market.