Ice storms are continuing to batter central U.S. states and have killed at least 21 people, the National Weather Service (NWS) said Monday.

Since last Friday, waves of freezing rain, sleet and snow have caused at least 12 deaths in Oklahoma, six in Missouri, two in Texas and one in New York, it said.

A state of emergency has been declared and the National Guard called out in Missouri after the storms knocked out power to 200,000 homes.

Churches cancelled Sunday services and businesses closed early or stayed shut.

The NWS said the storm system will head north-east, threatening to bring more than 30cm of snow to New England.

Ice storms occur when rain falls during very low temperatures, causing it to freeze almost instantly as it hits a surface, resulting in thick ice layers.

The extreme weather downed trees, traffic lights and power lines and blocked roads in Missouri and Oklahoma, where some 100,000 homes are without electricity.

There, about 350 national guardsmen went door to door to check on residents in the hardest-hit areas, and helped to clear icy roads of branches and power lines.

Oklahoma's governor, Brad Henry, has asked the federal government for assistance in handling the consequences of the storm.

In Texas, 415 flights were cancelled on Sunday at Dallas-Fort Worth International Airport.

Record-breaking cold weather even hit California, which usually has mild temperatures year-round.

Citrus fruit growers in parts of the state are using wind machines to protect their fruit from frost damage.

The NWS said it was "a very, very big cold front".

It said the storm is gathering moisture from the Gulf of Mexico and cold air from the Arctic, and is expected to hit the east coast on Tuesday.

Source: Xinhua