Texas Attorney General Greg Abbott urged people to stay away from the area as searchers look for survivors and investigators try to determine what caused the disaster, which authorities said killed an undetermined number of people.
Abbott warned unscrupulous business operators against price gouging for necessities.
"Last night was truly a nightmare scenario for that community," Gov. Rick Perry said during a news conference earlier in the day. "This tragedy has most likely hit every family and touched practically everyone in that town."
Texas Department of Public Safety spokesman Jason Reyes said he could confirm there were fatalities as a result of the explosions that shattered the town, but said the number had not been determined. Earlier, West Mayor Tommy Mouska put the number of dead at 40. Officials said about 160 people were injured.
Perry said he requested an emergency declaration from the federal government for McLennan County, where West is located, while lauding first responders and ordinary citizens who rushed to help victims, The Dallas Morning news reported.
"Last night was truly a nightmare scenario for that community," Perry said during a news conference Thursday. "This tragedy has most likely hit every family and touched practically everyone in that town."
The first explosion rocked the West Fertilizer Co. plant Wednesday evening. More explosions occurred several hours later.
Downtown sidewalks were littered with glass from shattered storefront windows. Homes crumbled to rubble, the Morning News said. The brick facade off a 50-unit apartment complex near the West Fertilizer Co. plant was blown away.
Among the missing were three or four members of West's volunteer fire department, including City Secretary Joey Pustejovsky, officials said.
Sgt. W. Patrick Swanton, police spokesman from the nearby Waco Police Department, said during a briefing Thursday authorities have "not gotten to the point of no return."
Houses and buildings destroyed or damaged by the blast were so frail that search teams must move slowly, Swanton said. Rescue workers shore up buildings as much as possible before they're searched.
Once the search is done, a building is marked to indicate no survivors are inside.
"It is a very slow and methodical search," Swanton said.
Meanwhile, state officials began their investigation into what sparked the explosion.
Nim Kidd, chief of the emergency management division with the Texas Department of Public Safety, said fighting a fire involving ammonium nitrate with water is a known hazard, but it was too soon to say what went wrong at the West plant.
President Barack Obama spoke to Perry Thursday morning.
"Today our prayers go out to the people of West, Texas, in the aftermath of last night's deadly explosion at a fertilizer plant," Obama said in a statement issued by the White House. "A tight-knit community has been shaken, and good, hard-working people have lost their lives."
He said his administration has been in close contact with state and local officials in Texas "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," Obama said.
The tragedy was mentioned by several speakers during an interfaith memorial service for Boston Marathon bombing victims.
The Morning News said the Czech ambassador to the United States was reported to be traveling to West, known for its annual Westfest celebrating the city's Czech heritage.
Swanton said the company site would be treated as a crime scene when officers gain full access. The cause of the explosions was unconfirmed.
"We are not indicating that it is a crime, but we don't know," Swanton said. "What that means to us is that until we know that it is an industrial accident, we will work it as a crime scene. ATF [the federal Bureau of Alcohol, Tobacco, Firearms and Explosives] is conducting the main investigation."
Department of Public Safety Trooper D.L. Wilson said the fire was still smoldering at the facility, noting caution was being exercised because of the proximity of flammable chemicals, the Waco Tribune-Herald reported.
Wilson said 50 to 75 houses were damaged and 133 people were evacuated from a nearby nursing home.
He described an apartment complex near the fertilizer plant as "a skeleton."
Wilson described the initial plant blast as "massive -- just like Iraq, just like the Murray Building in Oklahoma City. The same kind of hydrous [ammonia] exploded, so you can imagine what kind of damage we're looking at."
Half of the town of West, population about 2,800, has been evacuated, Wilson said. If the wind direction shifts, the other half may need to be evacuated.
The U.S. Geological Survey registered the blast as a 2.1-magnitude seismic event, ABC News said.