Rescue crews in the Midwestern US city of Joplin in Missouri have rummaged through piles of debris, searching for survivors of a massive tornado that killed at least 116 people and wreaked havoc in the city.
The rescue efforts on Tuesday came amid warnings that more powerful storms were on the way.
Joplin fire chief Mitch Randles put the death toll from the tornado at 116 people, but the state emergency management office said 117 had been killed, with 1,500 people missing.
Barack Obama, the US president, said he was "heartbroken" over the devastation wrought by the storm over the weekend and would visit the disaster zone on Sunday, following his four-nation visit to Europe.
"Our thoughts and prayers are with the families that are suffering at the moment," Obama told reporters in London, after earlier promising federal aid for recovery efforts.
"All we can do is let them know that all of America cares deeply about them and we are going to do absolutely everything we can to make sure they can recover," he said.
More than 1,000 other people were also injured in the twister that damaged or destroyed more than 2,000 buildings - or about a third of the city. Many of those wounded were reported to have massive internal injuries, according to local officials.
Al Jazeera's Rob Reynolds, reporting from the scene of the disaster in Joplin, said the death toll could still rise since some areas have not yet been searched by rescue teams.
The tornado struck the city near the border with Oklahoma and Kansas on Sunday evening, less than a month after a horrific tornado outbreak left 354 dead across seven US states.
The twister was the deadliest of 46 tornadoes reported to the National Weather Service in seven states on Sunday.
Scott Meeker of the Joplin Globe newspaper said the tornado turned the city into a "war zone".
"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," added Meeker, who is a resident of the town.
Many streets on the city's south side were described as impassible, littered with downed trees and utility poles. Emergency vehicles were racing across the city, taking injured residents to hospitals.
St John Regional Medical Center has evacuated over 183 patients after the hospital took a direct hit from the tornado, Cora Scott, a spokeswoman at the hospital's sister facility, said.
The patients were being taken to other hospitals. Witnesses said windows were blown out on the top floors of the hospital.
Al Jazeera's correspondent said several people in the hospital were killed when the storm hit and described the health care facility as "looking like it had been hit by a bomb".
Jeff Law, a resident of the city, was able to take shelter in a storm cellar and was overwhelmed by what he saw when he emerged from it after the tornado.
"I've lived in this neighbourhood my entire life, and I didn't know where I was," Law told the Springfield News-Leader. "Everything was unrecognisable, completely unrecognisable. It's like Armageddon."
Missouri governor Jay Nixon activated the National Guard and declared a state of emergency. Nixon said the state and local law enforcement agencies were co-ordinating search and rescue and recovery operations.
Tammy Spicer, a Missouri National Guard major, said more than 100 members of the 35th Engineer Brigade, which has a battalion based in Joplin, were expected to immediately report for duty to aid in search and rescue efforts, clear roads, provide security and help with radio communications.
The guard's armoury in Joplin suffered only minor damage but was without electricity Sunday night, Spicer said. One of the challenges facing the guard was simply getting in touch with all of its members in the area, she said.
"This just looks like a horrific event," Spicer said. "We want to help the community - both the emergency response forces there and the citizens."
Sunday's storms followed a tornado on Saturday night that swept through a small eastern Kansas town, killing one person and destroying at least 20 homes, as severe thunderstorms pelted the region with hail that some residents described as the size of baseballs.