A sixth person has died after tornadoes tore through the US Midwest at the weekend, emergency officials say.

Earlier three girls, aged five, seven and 10, and two men were among those killed in Woodward, Oklahoma, after the town's warning sirens failed to sound.

A flurry of twisters were also reported in Kansas, Iowa, and Nebraska as the storm front moved east.

Forecasters said dire warnings up to 24 hours in advance predicting severe storms had helped to save lives.

Tornado experts had said that storms on Saturday could be a "life-threatening event".

"We knew well ahead of time that this was going to be ugly. People listened," Tim Norton, Commissioner of Sedgwick County, Kansas, said.

He added that a "month's worth" of tornadoes had been spotted in Kansas over the weekend.

About 120 tornadoes reportedly touched down across the region on Saturday and Sunday.

Earlier, the National Weather Service (NWS) expressed concern that tornadoes could touch down in the early hours of Sunday morning and residents would not hear storm warnings as they slept.

Sirens are designed as outdoor warning systems and may not reach people in their homes, emergency management officials said, urging residents to receive warnings from a range of sources.

Reports suggest that lightning knocked out the tower that triggered the storm sirens in Woodward, causing the system to fail.

'Always devastating'

In Woodward, Frank Hobbie and his two daughters died when a tornado ripped through their trailer park. Darren Juul and a 10-year-old girl also died a few miles away.

One child who had been severely hurt was airlifted to a hospital in Texas, a spokeswoman from the State Medical Examiner's office said.

There was no information available about the sixth death, which was confirmed on Monday.

On Sunday rescue teams searched rubble for people trapped or wounded in Woodward, where high winds damaged homes, uprooted trees and brought down electricity lines. At least 29 people were injured.

A block of flats was also damaged by the twister, after residents were caught by surprise as the storm sirens had failed to sound, Reuters news agency quoted the local mayor as saying.

The governor of Oklahoma declared a state of emergency after the storms, paving the way for federal assistance with the clear-up.

"Our thoughts and prayers just go out to the families that have lost their loved ones, especially the children," Oklahoma Governor Mary Fallin said.

"It's always devastating to hear about the loss of life of children."

"This thing took us by surprise," Keli Cain, spokeswoman for Oklahoma Emergency Management, told the agency. "It's kind of overwhelming."

"They're still going door to door and in some cases there are piles of rubble and they are having to sift through the rubble," one of Ms Cain's deputies, Michelann Ooten, told the Associated Press.

In Iowa, a tornado destroyed large parts of the town of Thurman on Saturday, but there were no major injuries, the NWS said.

"It lasted three to four minutes probably - what seemed like an eternity," one man from Thurman told the broadcaster ABC.

"The next thing I know, the house was shaking and I could feel it lifting and it was over that quick," another man said.

Another twister caused widespread power outages and other damage in the city of Wichita, Kansas, AP reported.

The roof of a hospital in Creston, southwest of Des Moines, was reportedly damaged, but patients and staff were not hurt.

US tornadoes have already killed at least 39 people in 2012.

An outbreak of deadly twisters hit the states of Indiana, Kentucky, Ohio, Georgia and Alabama in early March.

At the start of April the Dallas-Fort Worth area was badly hit, with hundreds of flights being disrupted but no-one injured or killed.

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