BEIJING -- China's national observatory on Sunday evening renewed its orange alert for a cold wave as most parts of the country experienced the coldest weather in decades in the weekend.

China has a four-tier warning system for severe weather, with red being the most serious, followed by orange, yellow and blue.

From Sunday night to Monday night, temperatures in parts of south China will drop 3 to 6 degrees Celsius while the lowest temperatures in the lower reaches of the Yangtze River could reach minus 10 degrees Celsius, according to the National Meteorological Center (NMC).

Temperatures dropped 8 to 16 degrees Celsius from Thursday to Sunday in parts of northwest China, north China and the area between the Yellow River and Huaihe River, said the NMC.

Temperatures in central and eastern China were 6 to 8 degrees Celsius lower than the average historical level, the NMC added.

The southern city of Guangzhou saw rare sleet, the first in 60 years, in its downtown area, the provincial meteorological station announced on Sunday.

Zhejiang Province also recorded a historical low temperature of minus 20 degrees Celsius in mountain areas on Sunday. Temperatures dropped by as much as 15 degrees Celsius in the province.

In Hangzhou, capital of Zhejiang Province, civilian rescue groups participated in the emergency response. About 140 members of a group called the "Goat Team" have helped remove snow and defrost city streets.

The worst cold snap in years led to at least four deaths as it swept across central and southern China on Sunday.

In Changfeng County in east China's Anhui Province, four strawberry farmers died of carbon monoxide poisoning early Sunday morning when they turned up heating in a plastic greenhouse. Another six remain in the hospital.

The cold coincided with the beginning of a 40-day travel rush for the Spring Festival, which falls on Feb. 8 this year. Highway sections in northeastern and southeastern Yunnan Province, where snow and sleet have made travel impossible, have been closed.

Airport authorities and airlines are struggling to divert about 11,229 passengers who have been stranded at the Kunming airport since Saturday.

Snow and sleet will last through Monday in Yunnan.