US authorities say two people are confirmed dead after a tornado touched down in the north-west Oklahoma town of Woodward.
A sheriff's office dispatcher, Matt Jones, said two people were killed as the tornado hit part of Woodward and the nearby town of Tangier.
The National Weather Service said the tornado was reported at 12.18am local time (6.15am BST).
Woodward Mayor Roscoe Hill said sirens were apparently not working when the tornado hit. He said the sirens had been sounding loudly from storms.
Mr Hill said the tornado hit in a mixed area of residences and businesses.
Woodward police said search and rescue units had gone to assess the damage in the area.
The death toll later rose to five.
Mr Hill said: "We had a little tornado earlier... and they blew all the sirens. When this one came in, our sirens weren't working. We didn't have a very good storm alert."
Woodward, a town of about 12,000 people about 140 miles north-west of Oklahoma City, was among the many communities in the nation's midsection under a "high-end, life-threatening event" warning that had been issued by the Storm Prediction Centre in Norman, Oklahoma.