Emergency officials said local hospitals had treated some 550 people for injuries from flying debris in Oklahoma alone.
Fire departments urged local residents to be patient while efforts continued to find survivors and stabilize the situation, but many officials feared the death toll would rise as they continued their work.
"It is worse than what you can see," Bob Thompson, fire battalion chief in Sedgwick County, KS, told reporters. "We'll probably find more deaths. I don't think we've seen the end of it."
National Weather Service officials said up to 45 tornadoes touched down in Oklahoma in the last 24 hours and 14 struck in Kansas.
The largest of the tornadoes materialized approximately 45 miles southwest of Oklahoma City. The National Weather Service also said the tornado's winds measured some 250 mph, making it nearly a category F5 storm, the strongest measurable tornado. Most tornadoes only touch down for a short time, but the major twister in Oklahoma stayed on the ground for nearly four hours, traveling northwest for more than 70 miles through the Oklahoma City suburbs.
The cluster of twisters that occurred late Monday rank among the worst outbreaks of tornadoes in U.S. history, according to the National Oceanic and Atmospheric Administration (NOAA).
The governors of both Oklahoma and Kansas have declared the areas affected disaster areas and have requested federal assistance.
"We have whole communities that simply aren't there anymore," Oklahoma Governor Frank Keating told NBC's "Today" show. "It certainly looks like a huge battle has taken place."
On the federal level, the director of the Federal Emergency Management Agency toured the damage in Oklahoma, while President Clinton expressed his concern for those impacted by the storms.
"My heart goes out to the people of Oklahoma and Kansas who suffered through a night of terror and devastation," Clinton said in a statement. "Right now, our top priority is to make sure people are safe, that everyone is accounted for, and that initial cleanup can begin."
President Clinton approved federal disaster status in several counties affected. The declaration is aimed at speeding federal aid to hard-hit areas. In Oklahoma, it applies to eleven counties. In Kansas, the declaration affects Sedgwick County.
The combined death toll makes this the deadliest tornado outbreak since 1985 when 90 people were killed by storms in Ohio, Pennsylvania and Ontario.