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At least six people have been killed and thousands of others have been forced from their homes as wildfires continued raging Tuesday across hundreds of thousands of acres in Texas, Kansas and Oklahoma, authorities confirmed.

Fire activity was slowing across the region, but red flag warnings and fire weather watches, signifying serious fire conditions, were in effect into Wednesday afternoon from northern Iowa to northern Oklahoma, including almost all of Missouri and Kansas, the National Weather Service said.

"We are in a situation that can quickly become dangerous for the public and even firefighters due to the extreme fire weather conditions that can rapidly develop this time of the year," George Geissler, director of Oklahoma Forestry Services, told NBC station KFOR of Oklahoma City.

"As we continue to see normal firefighting tactics proving less effective and more time-consuming, we ask that everyone be aware of these conditions and use extreme caution with anything that can spark a fire," he said.

At least four people have been killed in Texas, authorities told NBC News, three of them in Gray County in the Texas Panhandle.

Flames and smoke envelope a grain elevator early Tuesday in Sitka, Kansas. Bo Rader / AP

Two were identified as Cody Crockett and his girlfriend, Sydney Wallace, both of whom were in their early 20s. The third victim was identified as Sloan Everett, about whom no information was immediately available.

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A man in his mid-20s whose identity was not disclosed was found dead in the road near Lipscomb, the Lipscomb County Sheriff's Office told NBC News. The man had gotten out of his vehicle and was overtaken by the fire, it said.

In Harper County, Oklahoma, an unidentified woman was killed after she had a heart attack while trying to evacuate cattle from the fires, emergency management officials told NBC News.

And in Clark County, Kansas, Corey P. Holt, 39, of Oklahoma City, died of smoke inhalation after he tried to back up his semi-trailer on state Highway 34 because of limited visibility from the fires, state Trooper Michael Racy told NBC News.

A plane drops fire retardant on a wildfire near Amarillo, Texas. Amarillo, Texas, Fire Department

Holt's truck ran off the road and jackknifed, authorities said. He was overcome by smoke when he got out of the truck, they said.

Crockett's sister, Callie, told NBC News that her brother and Wallace were helping Everett move cattle because of the fire in Gray County.

"He was on horseback, and they were in these sandhills, and they just got caught up in the midst of all the fire," Callie Crockett said.

"We are in a situation that can quickly become dangerous for the public and even firefighters due to the extreme fire weather conditions that can rapidly develop this time of the year."

Cody Crockett met Wallace when he was hospitalized for typhus two years ago, his sister said. Wallace was his nurse.

"We used to say 'thank God for typhus,' because that's how Cody met Sydney, and we love Sydney," Callie Crockett said.

Temperatures reaching into the 80s, low humidity and fierce winds are believed to have started most of the fires beginning overnight Sunday and Monday. The winds, topping 50 mph in some places, continued to severely hamper crews' efforts to stop the fires on Tuesday.

The largest fire, in Lipscomb and Ochiltree counties in the northeast corner of the Texas Panhandle, was at nearly 300,000 acres and was about 50 percent contained at noon Tuesday, the Texas A&M University Forest Service reported.

The fire in Gray County was estimated at 92,571 acres and was about 25 percent contained, the agency said.

In Reno County, Kansas, 10,000 to 12,000 people were evacuated from their homes because of a fire about 50 miles northwest of Wichita, the state Emergency Management Department said.

Kansas Gov. Sam Brownback said Tuesday that he had asked the governors of Wyoming and South Dakota to send extra equipment.

Meanwhile, another fire in Beaver County, Oklahoma, on the Texas border, was at 180,000 acres, the state Emergency Management Department said.