CHICAGO, May 23 (Xinhua) -- Authorities said on Tuesday that 122 people were killed in the tornado that battered Joplin, Missouri, on Sunday, making it the deadliest single U.S. tornado since the National Weather Service began modern record-keeping more than 60 years ago.

Joplin City Manager Mark Rohr estimated that 750 people have received treatment at area hospitals. He also warned that a new wave of dangerous storms is expected Tuesday evening.

Missouri Governor Jay Nixon said the toll is expected to climb as searching and rescuing continue. By Monday night, 17 people have been rescued. Joplin Emergency Management Director Keith Stammer said about 1,500 people are still unaccounted for.

The National Weather Service has determined the twister packed top winds of more than 200 miles per hour (322 km per hour), making it an EF-5 tornado on the enhanced Fujita scale.

 U.S. President Barack Obama said on Tuesday that he plans to visit the region this Sunday. Obama said he will let people know "the whole country is going to be behind them."

The tornado touched down into the heart of the city at around 6 p.m. (2300 GMT) on Sunday, destroying thousands of buildings. The local fire department estimated that 25 percent to 30 percent of the city was damaged by winds of up to 200 miles per hour, but the number of people   injured in the storm was still unknown. Jasper County emergency management director said 2,000 buildings were damaged.

The tornado even cut a path nearly six miles long and more than a half-mile wide in the downtown. Besides, a series of gas leaks caused fires around the city overnight.

A truck travels past a damaged building in tornado-hit Joplin in Missouri, the United States, May 24, 2011. The authorities said on Tuesday that the tornado that battered Joplin on Sunday had killed 122 people so far, making it the deadliest single U.S. tornado since the National Weather Service began modern record-keeping more than 60 years ago. (Xinhua/Zhang Baoping)

A local resident stands in front of a damaged building in tornado-hit Joplin in Missouri, the United States, May 24, 2011.  (Xinhua/Zhang Baoping)

Among the worst-hit locations in Joplin was patients-packed St. John's Regional Medical Center, which suffered a "direct hit" from tornado. The storm battered this nine-story building, blew out hundreds of windows and left the facility useless, with medical records, X-rays, insulation and other items found 60 miles away.

The city's residents were given about 20 minutes notice with tornado warning sirens before the tornado touched down on the city's west side.

But many people likely were unable to get to shelter in time, as "the storm was so loud you probably couldn't hear the sirens going off," said Missouri Governor Nixon.

The governor has declared a state of emergency and called out the Missouri National Guard to help. Authorities began a door-to-door search on Monday morning. But the task was made more difficult as a new thunderstorm with strong winds, heavy rain and "baseball"-sized hail continued to torment part of the city.

An official of Joplin Fire Department said the second search and rescue effort basically follows the path of tornado and "the third search is going to be similar to that. And then the fourth search through will be with the search-and-rescue dogs."

An official at the Federal Emergency Management Agency said Obama has issued a disaster declaration to expedite the dispersal of federal resources to the area.

More than 40 agencies were on the ground in the southwest Missouri city. Some 217 National Guard troops were on duty and 30 military police are helping with security and checkpoints as people try to return to their home in areas that may not be safe. Besides, the flood of aid from strangers and volunteers has helped ease the misery in Joplin.

The Joplin twister was one of 68 reported tornadoes across seven Midwest states over the weekend, spreading from Oklahoma to Wisconsin, according to the National Weather Service's Storm Prediction Center. The damage in Missouri was the worst of the day.

At least one person was killed in Minneapolis and a strong storm also caused significant damage in La Crosse, a city on the west of Wisconsin and there were no immediate reports of serious injuries.

The Storm Prediction Center said the high-risk area for severe storms includes southern Kansas, most of Oklahoma and southward into Texas. Four deaths were just reported in Oklahoma.

The worst for Joplin is expected to be over by Tuesday midnight, but tornadoes, large hail and damaging winds are still possible.

A small town in Kansas was stricken by a tornado on Saturday, which killed one person and destroyed at least 20 homes.

In the last month, tornadoes killed more than 300 people and caused an estimated damage of 2 billion U.S. dollars to the U.S. South. More than 200 were killed in Alabama alone.

Local residents collect useful items from a damaged building in tornado-hit Joplin in Missouri, the United States, May 24, 2011.  (Xinhua/Zhang Baoping)

A local resident stands in front of damaged buildings in tornado-hit Joplin in Missouri, the United States, May 24, 2011.  (Xinhua/Zhang Baoping)

Volunteers clean up the wreckages in tornado-hit Joplin in Missouri, the United States, May 24, 2011. (Xinhua/Zhang Baoping)

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