President Obama will speak to the nation at 10 a.m. Tuesday about relief efforts in Oklahoma, after a devastating tornado ripped through the Oklahoma City metropolitan area, killing dozens, including at least 20 children.

The president will be briefed by Homeland Security Secretary Janet Napolitano, chief of staff Denis McDonoughDenis Richard McDonoughEx-Obama chief of staff: Obama's Russia response was 'watered down' Former Obama officials launch advocacy group aimed at Trump's foreign policy Obama: Bannon, Breitbart shifted media narrative in 'powerful direction' MORE, deputy chief of staff Alyssa Mastromonaco and his assistant on Homeland Security and Counterterrorism Lisa Monaco before delivering a statement from the State Dining Room, the White House confirmed to The Hill.

The massive tornado, believed to be close to a mile wide, tore across the Oklahoma City suburbs of Newcastle and Moore on Monday afternoon, flattening homes and buildings, including two elementary schools. 

Hospitals said at least 145 people were injured, including 70 children. Gov. Mary Fallin (R) said emergency workers were focused on finding any survivors.

Obama on Monday declared a major disaster in the state and “ordered Federal aid to supplement state and local recovery efforts in the area affected by severe storms and tornadoes,” the White House said in a statement.

The measures would provide recovery funding for residents of five Oklahoma counties: Cleveland, Lincoln, McClain, Oklahoma and Pottawatomie. 

The White House said those affected would be eligible for “grants for temporary housing and home repairs, [and] low-cost loans to cover uninsured property losses.”

Federal Emergency Management Agency Administrator Craig Fugate is also headed to Oklahoma to help coordinate the relief efforts.

Rep. Tom Cole (R-Okla.), whose district was hit by the tornado, on Tuesday said the devastation was worse than the 1999 twisters that battered the same area.

"I never thought I'd see anything worse than what I saw in '99. This is even worse in terms of the loss of life — obviously we've got a high number now," Cole said on MSNBC's “Morning Joe.” 

The lawmaker said he feared the death toll would only rise in the days to come. "I've been talking to friends and family and officials on the ground. That number's going to get worse."

“The devastation is just unbelievable," said Cole.

The White House said President Obama had first been notified about the disaster shortly after the tornado hit and received updates from Monaco and Napolitano.

Obama and Napolitano also spoke with Fallin (R) on Monday, offering assistance from FEMA.

The governor said she was encouraged by Obama's offer of federal assistance, but cautioned that the recovery would be a long effort.

“He offered any type of assistance he could give. We appreciate the president, the administration, FEMA, approving our emergency declaration so quickly last night,” she said.

Napolitano said the administration would bring all available resources to bear to support state and local emergency management teams and first responders.

“Our thoughts and prayers go out to everyone impacted by the tornadoes and severe weather in Oklahoma, especially the families and loved ones of those killed, injured or missing," Napolitano said in a statement. "We encourage the public to listen to direction from state and local officials.”

This story was posted on May 20 at 6:43 pm and has been updated.