As many as 30 people are reported killed after a tornado tore through the city of Joplin in the US state of Missouri, officials say.
The town suffered a "direct hit" from the tornado and parts of the city have been devastated, local media says.
Missouri Governor Jay Nixon has declared a state of emergency and warned more storms are on the way.
Last month, tornadoes and storms killed at least 350 people in Alabama and six other southern states.
John Miller, a freelance photographer for the Springfield News-Leader newspaper, described widespread damage in Joplin.
"The Home Depot is levelled. The Walmart is destroyed. Gas stations, buildings. Everywhere I could see was either heavily damaged or completely destroyed," he said.
"I saw firefighters and paramedics pull a young girl out of a car at the Home Depot. Part of the building had fallen on the car."
Hospital damaged
Power lines and phone lines in the city are down, reports say.
Witnesses said ambulances lined the street near a Walmart store amid reports that hundreds of people were trapped inside.
Officials at St John's Hospital in Joplin said the building had been badly damaged and windows blown out. A resident living 45 miles (70km) away said debris from the hospital had landed in his yard, including medical supplies and X-rays.
Newton County Coroner Mark Bridges told Reuters by telephone that at least 30 people had been killed.
The Red Cross has opened a shelter at Missouri Southern State University in Joplin for victims of the disaster, Red Cross spokeswoman Joanne Muir told the BBC.
It has also sent an emergency response vehicle with some supplies such as blankets, cots, water and food to the area.
"I know the hospital did take a direct hit and I have heard from people that some people have lost their homes, and there was damage to one of the schools," she said.
Continued risk
US President Barack Obama - on his way to the Republic of Ireland - sent his condolences to those affected.
"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.
"We commend the heroic efforts by those who have responded and who are working to help their friends and neighbours at this very difficult time."
Governor Nixon said storms had caused extensive damage across Missouri.
"They continue to pose significant risk to lives and property," he said in a statement.
"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.
He warned that the storms were not finished.
"I urge Missourians to keep a close eye on the latest weather information and to follow the instructions and warnings of emergency personnel as these deadly storms continue to move through our state," he said.
