(AP Photo/Charlie Riedel)
At least 89 people have been confirmed dead in the tornado that struck Sunday.
"I don't think we're done counting," Nixon told The Associated Press. But he added: "I still believe that because of the size of the debris and the number of people involved that there are lives to be saved."
Nixon was heading to Joplin on Monday to view the damage and meet with local officials.
The Joplin tornado appears to be the worst in Missouri in many decades.
According to the federal Storm Prediction Center, an estimated 255 people were killed when a tornado struck St. Louis on May 27, 1896; an estimated 99 people died from an April 18, 1880, tornado that struck Marshfield; and an estimated 98 people were killed by a tornado hit Poplar Bluff on May 9, 1927.
Missouri also was involved in the nation's deadliest, multi-state string of tornadoes, when an estimated 695 people were killed in Missouri, Illinois and Indiana on March 18, 1925.
The search for survivors was the main focus Monday for a rescue effort that Nixon said included 40 agencies from Missouri, Arkansas, Kansas and Oklahoma. The governor said about 2,000 buildings suffered significant damage, including St. John's Regional Medical Center, a Walmart and numerous other businesses, homes, churches and schools.
Nixon said he had received reports of several rescues from the rubble of stores but didn't know where the greatest number of causalities had occurred in Joplin. About 400 people had been treated at Freeman Health System in Joplin and more than 100 people had spent the night at a shelter -- numbers which were likely to rise, the governor said.
Nixon spoke overnight with Homeland Security Director Janet Napolitano and Monday morning with President Barack Obama. The president has directed Federal Emergency Management Agency Administrator Craig Fugate to travel to Missouri to ensure the state has all the support it needs, said White House spokesman Nicholas Shapiro.
"Getting an accurate count of who's missing and who's not and searching directly is the number one priority this morning," Nixon said.
Residents in the Joplin area had about 17 minutes of warning before the tornado hit, but "there was so much rain, so much wind around it, it was -- I think -- very difficult to hear even the sirens," Nixon said.
Associated Press writer Chris Blank contributed to this report.