A number of tornadoes ripped through parts of the U.S. Midwest on Tuesday, even as the city of Joplin, Miss., struggled to recover from the deadliest tornado to strike the United States in more than 60 years.

Oklahoma Gov. Mary Fallin told CNN that the state was hit by at least four tornadoes. Authorities said at least four people were killed after a tornado touched down in an Oklahoma City suburb. It was not clear how they died. Another woman died at a mobile home park in Oklahoma.

Three children were in critical condition after the storms hit, and search dogs were called out to search for victims in Canton.

The storms began about 3 p.m. in western Oklahoma. One of the tornadoes that was on the ground swept through the towns of El Reno and Piedmont in the western and northern Oklahoma City suburbs.

Emergency workers told the Associated Press that a number of people were injured at Piedmont, a small community northwest of downtown Oklahoma City.

In Kansas, police said two people died when high winds threw a tree into their van around 6 p.m. near the small town of St. John, about 150 kilometres west of Wichita. The highway was shut down because of storm damage.

One person was reported dead early Wednesday after a twister hit the Arkansas community of Denning.

On Sunday, a single twister in Joplin killed at least 122 people. More than 750 were injured. That's the highest toll from one tornado since accurate record-keeping began, surpassing the 116 people who were killed in a single twister in Flint, Mich., on April 27, 1953.

And the Joplin figure is expected to rise.  The National Weather Service said the twister had winds greater than 320 km/h and was an EF5 on the Fujita scale — the highest rating assigned to tornadoes based on the damage they cause. 

Gathering storms

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The huge thunderstorm that spawned the Joplin tornado formed over southeastern Kansas on Sunday afternoon, along the boundary between warm, moist air flowing northwards from the Gulf of Mexico and cold, dry air moving south from Canada.

Tornadoes can occur during any month in the U.S. But May, June and July are the most active months, with an average of more than 800.

While the numbers this year are only preliminary, more than 1,000 tornadoes were reported up to the end of April. This could turn out to be one of the worst years on record for tornadoes and severe weather. 

Here's what's happening: During the peak months — in late spring and early summer — cool, dry air clashes with humid, warm air right over the Ohio and Mississippi valleys. This year that combination has been particularly extreme, especially when you throw in strong winds at varying altitudes that help to ignite the storms.

The ingredients are ripe again Tuesday for more thunderstorms. A large section from Kansas to Texas is at risk of severe weather today.

Johanna Wagstaffe

Though the storm has passed, danger was by no means over. Fires from gas leaks burned across Joplin on Tuesday. The smell of ammonia and propane filled the air in some damaged areas. Unstable buildings also posed threats, as whipping winds threaten to finish them off.

Joplin Mayor Mark Rohr told a media briefing Tuesday morning that the search-and-rescue effort was continuing while the weather remained good, with search dogs and handlers coming "from all over."

Fire Chief Mitch Randles said searchers were still finding survivors and bodies. Seven survivors were found Monday, including one inside the wreckage of a Home Depot outlet.

Asked about looting, a spokesman for the state police said there had been isolated problems, but they had been taken care of. Police are checking everyone leaving the area, he said.

He urged sightseers to stay away. "Yesterday, there was a problem with sightseers coming in," he said. "Every news station is streaming this live and on the internet. If you want to watch and see the destruction, watch it on TV."

After a record-breaking April, May is shaping up to be a busy month for tornadoes. Preliminary reports suggest more than 1,000 tornadoes touched down in April alone.

The twister that hit Joplin was one of more than 50 reported across seven Midwestern states over the weekend. One person was killed in Minneapolis and another in Kansas, but Missouri took the hardest hits.

Flash-flooding warnings remained in place around Joplin through Tuesday after more thunderstorms Monday.

Amid the despair, there were glimmers of hope: rescuers pulled 17 people from the rubble, and Missouri Gov. Jay Nixon vowed that crews would keep searching until everyone is accounted for.

"They still think there are folks that could be alive," Nixon told The Associated Press. Searchlights were brought in for work to continue overnight.

U.S. President Barack Obama plans to visit Joplin on Sunday. He pledged Tuesday that the federal government will use all resources at its disposal to help Midwesterners recover and rebuild after a "devastating and heartbreaking" series of storms.

"I want everybody in Joplin, everybody in Missouri, everybody in Minnesota, everybody across the Midwest to know that we are here for you," Obama said in London.

"The American people are by your side. We're going to stay there until every home is repaired, until every neighbourhood is rebuilt, until every business is back on its feet."

Obama spoke from London, where he is on the second stop on a four-country, six-day tour of Europe. He has already made a brief visit to Ireland, and he is to appear at the G8 summit in France at the end of the week.

He is due back in Washington on Saturday night.

With files from The Associated Press