A massive tornado cut a deadly swath through a Missouri town Sunday, turning homes into rubble, destroying the local high school, ripping huge chunks out of a hospital and reportedly killing at least 24 people.

The tornado struck the town of Joplin near the border with Oklahoma and Kansas less than a month after a horrific tornado outbreak left 354 dead across seven US states.

It was the deadliest of 46 tornadoes reported to the National Weather Service in seven states Sunday.

"It's a warzone," Scott Meeker, enterprise editor with the Joplin Globe, told AFP.

"We've got hundreds of wounded being treated at Memorial Hall (hospital), but they were quickly overwhelmed and ran out of supplies so they've opened up a local school as a triage center."

People clawed through the rubble looking for friends, family and neighbors after the late afternoon storm tore buildings apart and turned cars into crumpled heaps of metal.

Flames and thick black smoke poured out of the wreckage of completely unrecognizable homes and water gushed out of broke pipes as shocked survivors surveyed the damage, early photos showed.

A tangled medical helicopter lay in the rubble outside St. John Regional Medical Center, which took a direct hit.

Jeff Law, 23, was able to take shelter in a storm cellar and was overwhelmed by what he saw when he emerged.

"I've lived in this neighborhood my entire life, and I didn't know where I was," Law told the Springfield News-Leader. "Everything was unrecognizable. Completely unrecognizable. It's like Armageddon."

The emergency manager at the neighboring county of Springfield-Greene County was told at least 24 people were killed before he rushed over to help, a spokeswoman told AFP.

With many phones down in the area, it was difficult to get further confirmation.

A man who answered the phone at the county coroner's office said he didn't have an official toll yet.

"It's still a disaster area out there," he told AFP.

"We're jut trying to find everyone."

Missouri Governor Jay Nixon declared a state of emergency and activated National Guard troops in response to what he described as "significant destruction in multiple areas, including Joplin, where a tornado struck St. John's Regional Medical Center."

The badly damaged medical center was evacuated Sunday and Nixon warned that the storms are not finished.

"These storms have caused extensive damage across Missouri, and they continue to pose significant risk to lives and property" Nixon said in a statement late Sunday.

"As a state, we are deploying every agency and resource available to keep Missouri families safe, search for the missing, provide emergency medical care, and begin to recover," he added.

President Barack Obama sent his "deepest condolences" to victims and said the federal government stood ready to help Americans as needed.

Michelle and I send our deepest condolences to the families of all those who lost their lives in the tornadoes and severe weather that struck Joplin, Missouri as well as communities across the Midwest today," the president said in a statement sent from Air Force One as he was flying to Europe.

"We commend the heroic efforts by those who have responded and who are working to help their friends and neighbors at this very difficult time," the president added.

On Saturday, a deadly tornado pummeled the east Kansas town of Reading, killing a man and damaging an estimated 80 percent of Reading's structures, mostly wood-frame buildings.

According to witnesses, it crushed a grain elevator, tore the top off the red brick post office, blew the back off the local fire department building, tore houses off foundations and uprooted trees.

"We have nothing, no place to call home. It's hard," Jill Scales, a Reading resident, told reporters. "We're just still in shock. We don't know where to go from here. What do you do?"

A tornado was also responsible for the death of one person in Minneapolis, Minnesota, on Sunday, authorities said. At least 18 others in that city and its suburbs were injured.