Unseasonably cold weather and blizzards hit northeast China on Monday, forcing hundreds of flights to be rescheduled and closing schools as several cities issued heightened weather alerts and warned people to stay indoors.

As of 10:30 a.m. (0230 GMT), 405 flights were cancelled at Harbin Taiping International Airport in the capital of Heilongjiang, the country's northernmost province, according to data from third-party travel app Flight Master.

Earlier, Heilongjiang's government said operations were otherwise normal at the airport.

Most parts of Harbin suspended primary and secondary schools, kindergartens, and off-campus training institutions, according to the government's official WeChat account, as snow and freezing temperatures hit the region.

Many flights were also cancelled in Jilin and Liaoning provinces, and in the Inner Mongolia region, Flight Master showed.

China's weather authority warned of a drastic drop in temperature in coming days, along with blizzards, anticipated to substantially affect several cities, state media reported.

"The season changed outside overnight," a netizen from Heilongjiang complained on Weibo, China's popular social media platform.

Provinces and cities upgraded weather response protocols as heavy snowfall is expected in parts of Inner Mongolia, and Hebei, Jilin and Liaoning provinces, China Daily reported.

On Monday, Inner Mongolia, and Liaoning and Jilin provinces suspended schools.

Late on Sunday, Heilongjiang issued a red alert - the country's highest weather advisory - as cities in the province were expected to see 20 mm (0.787 inches) to 40 mm (1.575 inches) of blizzard precipitation into Monday evening, according to Chinese weather forecasters, China Daily reported.

The Central Meteorological Observatory said some areas could see 8-10 cm (3.14 to 3.94 inches) of snow and temperatures in many regions would drop sharply.

Chinese weather forecasters kept orange alerts for blizzards in several areas, while China's National Meteorological Center issued an orange alert for blizzards and a blue alert for cold waves and strong wind, Global Times reported.

China has a four-tier colour-coded weather alert system, with red the highest, followed by orange, yellow and blue.

The cold forced governments in Beijing and Tianjin to make preparations to supply heat earlier than usual, according to city government notices.

Over the past week, northern China has experienced unseasonably odd contrasts in weather from smog to logging the second-warmest October in decades and then a sharp drop in temperatures over the weekend.

The Central Meteorological Administration said this week most of the northeast is expected to see temperatures dive to the single digits or below freezing as cold air moves east and south. (Reporting by Bernard Orr, Ella Cao and Shanghai newsroom; Editing by Raju Gopalakrishnan)