Deadly tornado ravages Missouri

A massive tornado has cut a deadly swath through a Missouri city, 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, which struck the city of Joplin near the border with Oklahoma and Kansas, was the deadliest of 46 tornadoes reported to the National Weather Service in seven American states on Sunday.

Asked about a report that 24 people had died, Joplin city spokeswoman Lynn Onstot said grimly that officials were "afraid it may be more", the Associated Press reported.

"Our fear is that's a low number," she said.

"It's a warzone," Scott Meeker from the Joplin Globe newspaper told Agence France-Presse.

"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 centre."

People clawed through the rubble looking for friends, family and neighbours 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 unrecognisable 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.

The hospital staff had just a few moments' notice to hustle patients into hallways before the storm struck the multi-storey building, blowing out hundreds of windows and leaving the facility useless.

Triage centres and shelters were set up around the city of about 50,000 people, which is about 260 kilometres south of Kansas City.

Emergency management officials rushed heavy equipment to Joplin to help lift debris and clear the way for search and recovery operations.

Missouri Governor Jay Nixon declared a state of emergency and activated National Guard troops, and US President Barack Obama sent condolences to families of those who died in storms in Joplin and across the Midwest.

The same storm system that produced the Joplin tornado sparked tornadoes from Oklahoma to Wisconsin.

At least one person was killed in Minneapolis. But the devastation in Missouri appeared to be the worst of the day, eerily reminiscent of the tornadoes that killed more than 300 people across the US south last month.

"You see pictures of World War II, the devastation and all that with the bombing. That's really what it looked like," said Kerry Sachetta, the principal of a flattened Joplin High School.

"I couldn't even make out the side of the building. It was total devastation in my view. I just couldn't believe what I saw."

Onstot said the twister was on the ground for nearly six kilometres.

It hit the hospital and a commercial area including a construction store, numerous smaller businesses and restaurants and a grocery store.

An untold number of homes were destroyed and reduced to ruin.

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

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

"It's so devastating we can't even grasp it at this point," Rob Chappel of the Jasper County coroner's office told AFP.

"We're still trying to rescue victims that are still trapped. With it being so dark and no electricity, everyone has underestimated how much is just gone."

Chappel said authorities probably wouldn't know the exact death toll before late Monday at the earliest.

Governor Nixon warned that the storms were 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 national government stood ready to help Americans as needed.