BEIJING - Millions of families in northern China have been forced to give up coal for their winter heating as part of Beijing's campaign to fight smog, but some are now left out in the cold - literally.

Since August, the Chinese government has poured tens of billions of yuan into a massive project to install equipment, build thousands of kilometres of pipes and subsidise the higher costs of gas. Beijing alone has closed its four major coal-fired power plants and replaced them with four gas-fired plants.

The goal is to have millions of people in China's frigid north bid goodbye to coal in favour of cleaner alternatives from the beginning of this month (Nov), when temperatures start to plunge.

Beijing has been under increasing pressure to deal with chronic air pollution amid concerns about the damage it is causing to people's health. Smog gets worse during the colder months when homes in the north of the country crank up heat that is overwhelmingly fired by coal, according to Reuters.

But delays in the construction of new pipelines to supply natural gas meant that some households have neither coal nor natural gas to cook, bathe, and keep warm during the coldest months of the year.

One woman in Linfen, Shanxi province, who declined to be identified, said all of the boilers in her village had been dismantled, but work on new gas pipes appeared to be nowhere near completion, reported South China Morning Post.

The 30-year-old said the local Communist Party committee had warned residents against burning coal for heating with the slogan: "If your home has smoke coming out, see you in the detention centre."

"They're still doing the digging work to lay the gas pipes," she said. "My baby has fallen ill because of the cold."

Apart from the delays in setting up the natural gas pipes, many northern cities also face a severe supply shortage of natural gas as more companies and households switch to the cleaner fuel.

In Taishan, Shandong province, local supplier Taishan Gas Group said thousands of households had their gas cut off over the weekend because of insufficient supply.

On social media site Weibo, dozens of users from northern China said they had yet to be provided with the natural gas promised by the authorities.

"I can't bear the cold in my home, even with a thick coat on," a user in Yongqing county, Hebei province wrote on Sunday.

"We don't have any heating on such cold days," a user from Anyang, Henan also wrote on Sunday.

"How will we get through the winter?"

Temperatures are expected to fall in the coming weeks.

In many houses, radiators powered by natural gas have replaced systems that have been used for centuries in rural villages in northern China - burning coal to heat large beds where whole families gather during the winter.

Yet some locals of Shanxi province, home to 40 per cent of China's total coal reserves, told Sixth Tone they are not confident that the new technologieswill improve their living conditions.

Villager Zhao Xiaoxiao believes electricity will prove cleaner than coal, but she doubts it will be make the homes warmer.

"The structure of our houses is different from apartments in the city - they're not well-insulated," she says. In the winter, temperatures in her village drop as low as minus 20 deg C.

China's winter temperatures fell to a record low of minus 47.8 deg C in Genhe city in Inner Mongolia region last year (2016).

Even for those families whose new radiators are up and running, they are worried about the higher costs.

Farmer Zhang Peiying from Shanxi province told Sixth Tone news website: "Using gas will be cleaner."

But "nobody has told me how much the heating bill will be", she said.

Her family used to pay less than 1,000 yuan on average during the five coldest months of the year when they relied coal for winter heating.

Natural gas is up to three times more expensive than coal, reported Sixth Tone.

Pofessor Li Xiangfang, a professor of natural gas engineering at China University of Petroleum in Beijing, told Sixth Tone that the cost of gas was already higher than it should be due to a lack of supply.

"In recent years, China's supply of natural gas has not been able to meet the rapidly increasing demand," Li says.

China has become one of the world's largest importers of natural gas, with 36.6 per cent of its resources coming from abroad.

"The switch from coal to natural gas will boost the demand for gas," Li added.

"The price of natural gas, especially in the countryside, should be further lowered."