When the fertilizer plant exploded in West, Texas last night “it was like a small nuke,” said a senior Obama administration about the force of the explosion and the damage it caused.

Although exact fatality numbers haven’t been released by authorities in Texas, the administration official said there are fears that the death toll could be much higher than the 15 people speculated about since the explosion took place.

Authorities suspect the blast was set off by a rail car holding a large quantity of ammonia nitrate that somehow caught fire or blew up. The Obama offical told The Post that investigators are now trying to determine whether the ammonia nitrate-carrying train was on the fertilizer plant’s property or on an adjacent site.

That question is critical, the official said, because the plant does not list ammonium nitrate as a chemical that it handles under the plant’s emergency plan on file with federal regulators — as it should be if the plant does handle such a chemical.

“There’s no ammonium nitrate reporting on their emergency plan,” the official said, noting that that plan was most recently reviewed by regulators in 2011.

In 2004, the facility was cited by federal regulators for its lack of a required emergency plan, the official said. The facility had rectified that failure by 2006.

Regarding the fire and subsequent explosion, the official said, “It’s believed right now that it is an industrial accident,” adding that authorities have essentially ruled out the possibility that the conflagration was an act of terrorism.

“Something caught fire, and therefore lead to the ammonium blast,” the official said.

“But it still could be criminal,” the official said, pointing out that if someone had been negligent or reckless in storing or handling unapproved chemicals on the site, they could face prosecution.

Because of the ammonium still in the air, investigators have been unable to get access to the blast scene, and figure out where exactly the explosion began.

But air monitors in the surrounding community are showing safe readings, because there is a northwestern wind blowing chemicals away from the residential areas, the official said.

Earlier in the day Texas Govenor Rick Perry declaired McLennan County a disaster area in the wake of the explosion.

“Last night was truly a nightmare scenario for that community,” Perry said at a news conference today. “I ask all Texans and Americans to join me and [my wife] Anita in keeping them in our prayers.”

Perry also said that President Obama has offered a quick turnaround in getting federal aid to the disaster area.

Meanwhile, Waco Police Sgt. William Patrick Swanton said officials on the ground remain in “search and rescue mode,” going building to building in the largely decimated neighborhood surrounding the plant.

He says he knows some firefighters still are missing.

Swanton says a thunderstorm that rolled through the area early Thursday has helped in some ways, including tamping down chemicals released from the plant.

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President Obama said in a statement today, “Today our prayers go out to the people of West, Texas in the aftermath of last night’s deadly explosion at a fertilizer plant. A tightknit community has been shaken, and good, hard-working people have lost their lives. I want to thank the first responders who worked tirelessly through the night to contain the situation and treat the wounded.

“My Administration, through FEMA and other agencies, is in close contact with our state and local partners on the ground to make sure there are no unmet needs as search and rescue and response operations continue. West is a town that many Texans hold near and dear to their hearts, and as residents continue to respond to this tragedy, they will have the support of the American people.”

“There was an apartment complex that was completely destroyed. It looked like a bomb blast in Iraq,” said Texas state trooper spokesman D.L. Wilson, who told The Post that the blast injured “hundreds” of people and damaged 75 homes.

“There were 133 victims in the nursing home that we had to get out,” Wilson said.

“A lot of elderly people were injured by flying bricks.”

Swanton said the death toll is still being calculated but that somewhere between five and 15 people have died so far.

“I think we will see the number of fatalities increase as we reach the morning,” he said.

“We’re going to hope for the best and prepare for whatever we come across.”

SOCIAL MEDIA REACTION TO EXPLOSION

PHOTOS: TEXAS TRAGEDY

REFLECTING ON THE 1947 TEXAS CITY DISASTER

West Mayor Tommy Muska said buildings in a five-block radius from the plant were severely damaged by the explosion.

“We need your prayers. We’ve got a lot of people who are hurt, and there’s a lot of people, I’m sure, who aren’t gonna be here tomorrow,” Muska said.

“We’re gonna search for everybody,” he vowed. “We’re gonna make sure everybody’s accounted for. That’s the most important thing right now.”

West EMS director George Smith was helping to rescue residents when the plant exploded.

“The windows came in on me, the roof came in on me, the ceiling caved,” he said in an interview with KCENTV.com.

The plant blew up after a fire that burned earlier in the day rekindled, sparking the explosion shortly before 8 p.m., Wilson said.

Ambulances, private vehicles and buses rushed victims to hospitals in Waco, about 20 miles north of the plant.

“Currently, we have treated 34 patients and we have not had any fatalities yet,” said Heather Beck, a spokeswoman for Providence Healthcare Network.

“We actually have a bus coming right now with 15 more patients.”

By morning, Providence had treated nearly 70 patients. Other local hospitals treated hundreds of other injured patients, the blast tearing a hole through the tight-knit Texas community.

The explosion was so powerful it was felt 45 miles away – and registered as a 2.1-magnitude earthquake with the United States Geological Survey.

“It was like being in a tornado. Stuff was flying everywhere. It blew out my windshield,” said Debbie Marak, 58, who was driving near the plant when it exploded.

“It was like the whole earth shook.”

With Post Wire Services