"The generosity of Oklahomans, Americans and people across the world is very encouraging and will help meet many of the short-term needs of victims," Fallin said in a statement. "However, experience tells us there also will be long-term consequences to the challenges victims are facing."
Those wishing to donate can call (405) 236-8441 or go online to unitedwayokc.org.
[Updated at 4:41 p.m. ET]
Diplomats in Geneva, Switzerland, opened a United Nations meeting on disaster risk reduction with expressions of sympathy for those impacted in Oklahoma.
"The impact of this disaster was evident for one of the world's most economically developed countries," said Deputy Secretary-General Jan Eliasson. "Think how much more dangerous the situation is in places where people are poor and living in fragile homes with insufficient water and health services."
[Updated at 4:28 p.m. ET]
Damage assessments show that the tornado gained significantly in strength -- from an EF0 to EF4 -- over a 10-minute span, the National Weather Service reports.
The tornado that hit Moore tornado was 1.3 miles wide, according to the weather service. Its estimated top winds were between 200 and 210 mph, putting it in the EF5 category -- the strongest possible for a tornado.
[Updated at 4:12 p.m. ET]
Stephen Eddy, city manager for Moore, told CNN's Jake Tapper that "everyone has been found" who was believed missing because of the devastating twister. He also expressed optimism that his central Oklahoma city would rebound.
"We've been through this before," Eddy said. "We've come back stronger than before every time."
[Updated at 3:58 p.m. ET]
The National Weather Service's Norman, Oklahoma, office reported that "at least one area of EF5 damage was found by survey crews."
[Updated at 3:47 p.m. ET]
Tributes continue to pour in for those teachers who helped protect children as the tornado barreled through Oklahoma. U.S. Education Secretary Arne Duncan was among those saluting them, tweeting that he is "inspired by the selfless teachers and staff who protected children from harm in Oklahoma."
[Updated at 3:44 p.m. ET]
Moore Mayor Glenn Lewis said authorities there will probably push for even more measures -- on top of those already in place -- to protect buildings against tornadoes.
He also thanked state and federal authorities for responding speedily, and extensively, in the wake of the devastating tornado.
"They were Johnny-on-the-spot," Lewis told CNN's Anderson Cooper. "And they've sent tons of help."
[Updated at 3:40 p.m. ET]
The Oklahoma City Thunder and its charitable foundation together are donating $1 million to the Red Cross, Salvation Army and other disaster relief organizations to help in the aftermath of this week's tornado.
"We are focusing Thunder resources to help where we can in the relief efforts and to support the organizations that are on the ground assisting those affected by this week's storms," the NBA team's chairman, Clay Bennett, said. "Even with so much loss, the strength and resiliency of this community have once again been on display, and we will continue to work together as our community and state recover from this disaster."
The NBA and National Basketball Players Association have also pledged $1 million.
[Updated at 2:50 p.m. ET]
The estimated peak wind in the tornado was 190 mph, the National Weather Service said on Tuesday afternoon. That still is a preliminary estimate, according to the weather service. The estimate would make the tornado, as the weather service preliminarily said yesterday, an EF4 on the Enhanced Fujita scale (meaning it had winds between 166 and 200 mph).