Four adults and two children were killed when a tornado hit Madison County in the US state of Iowa on Saturday, officials said.

At least four other adults were injured in the tornado that caused "widespread damage to both homes and properties," the county's emergency management director told a press briefing.

"This is, I think, the worst that anyone has seen in quite a long time," Diogenes Ayala said. "This will be impactful for many years to come."

National Weather Service Des Moines tweeted at 7:30 pm (0130 GMT Sunday) that "the worst is over for central Iowa." 

"Storms are moving off to the East. Areas west of the line, including the Des Moines metro, are in the clear."

NWS said its survey teams would thoroughly investigate the damage on Sunday.

"This is pretty devastating for our community," Ayala said, adding that rescue efforts would continue overnight.

Iowa Governor Kim Reynolds issued a disaster proclamation for Madison County, which allows state resources to be used to assist with recovery. 

"Our hearts go out to all those affected by the deadly storms that tore through our state today," Reynolds said in a statement. 

In December, dozens of devastating tornadoes ripped through five US states overnight, leaving at least 79 people dead in Kentucky -- with fatalities also recorded in Tennessee, Arkansas, Missouri and Illinois.