Schools in the Chinese capital were ordered Sunday to keep children indoors in the coming days, when the city's notorious choking smog is forecast to return.

All kindergartens, elementary and middle schools must suspend outdoor activities after the city issued another "orange" alert for smog from Monday to Wednesday, the official Xinhua news agency reported.

The decision was taken Sunday by the city's Commission of Education.

Beijing first issued an orange alert -- the second-highest warning -- last week, ordering hundreds of factories to shut and allowing children to skip school. Construction sites were told to stop activities which create dust.

The smog, which at one point was over 25 times safe levels and covered swathes of northern China, cleared later in the week.

Some schools allowed students to choose whether to study at home or in school while Beijing Digital School ran classes online, the education commission said.

The smog is created largely by vehicle exhaust emissions and coal-burning by industry and for winter heating.

The frequently filthy air is a major source of public discontent with the Communist rulers.

China pledged last week to upgrade the nation's coal-fired power plants to cut pollution, as negotiating teams were locked in crucial talks at a climate change summit in Paris.

The State Council, or cabinet, announced plans to reduce by 60 percent the amount of "major pollutants" coming from its coal-fired power plants by 2020.

Analysts said the announcement was intended mainly to allay public anger over smog rather than to tackle greenhouse gas emissions.