According to a statement sent by the Ministry of Environmental Protection (MEP) on Sunday to the Global Times, activities in some Beijing suburbs violated the contingency plan, including the failure to suspend construction work and the illegal burning of garbage.

The MEP also said Beijing has more than 5.7 million vehicles, and the nitric oxide and hydrocarbons emitted from them account for 50 and 40 per cent of the smog.

Pollution created by out-of-town vehicles was more severe, with about 300,000 vehicles entering Beijing daily, with half of them exceeding emissions standards, said the statement.

Meantime, five illegal trash-burning areas were found around Beijing, which has contributed to the smog, the statement said.

Beijing Emergency Management raised the pollution alert level from blue to yellow on Friday, the third-highest warning level, because the density of PM 2.5 particles reached 350 micrograms per cubic metre and worsened on Saturday, China News Service reported.

The heavy pollution that blanketed Beijing the past days was mainly caused by local pollutants since PM 10 particles were higher than before, according to monitored data.

The report said clear skies are expected Monday morning due to the cold front, but will deteriorate again on Tuesday because of rising humidity, among other reasons.

The Beijing Municipal Transport Commission recently issued draft regulations on car-booking services, stating that only those with a Beijing household registration with a car of certain engine displacements are qualified to provide the service.

Authorities said the measure is aimed at easing traffic as well as reducing vehicle emissions.

Many cities in the Beijing-Tianjin-Hebei region have suffered mild to moderate air pollution since Saturday, and some places may suffer from heavy pollution from Tuesday to Thursday.