PHILADELPHIA — An elevated section of Interstate 95 collapsed early Sunday in Philadelphia after a tanker truck carrying flammable cargo caught fire, closing a heavily traveled segment of the East Coast's main north-south highway indefinitely, authorities said.

Transportation officials warned of extensive delays and street closures and urged drivers to avoid the area in the city's northeast corner. Officials said the tanker contained a petroleum product that may have been hundreds of gallons of gasoline. The fire took about an hour to get under control.

The northbound lanes of I-95 were gone and the southbound lanes were "compromised" by heat from the fire, said Derek Bowmer, battalion chief of the Philadelphia Fire Department. Runoff from the fire or perhaps broken gas lines caused explosions underground, he added.

Some kind of crash happened on a ramp underneath northbound I-95 around 6:15 a.m., said state Transportation Department spokesman Brad Rudolph, and the northbound section above the fire collapsed quickly.

The southbound lanes were heavily damaged, "and we are assessing that now," Rudolph said.

Gov. Josh Shapiro, who said Sunday evening he planned to issue a disaster declaration Monday to speed federal funds, said at least one vehicle was still trapped beneath the collapsed roadway.

"We're still working to identify any individual or individuals who may have been caught in the fire and the collapse," he said. There were no immediate reports of injuries.

Video from the scene showed a massive concrete slab had fallen from I-95 onto the road below. Shapiro said his flight over the area showed "just remarkable devastation."

"I found myself thanking the Lord that no motorists who were on I-95 were injured or died," he said.

Mark Fusetti, a retired Philadelphia police sergeant, said he was driving south toward the city's airport when he noticed thick, black smoke rising over the highway. As he passed the fire, the road beneath began to "dip," creating a noticeable depression that was visible in video he took of the scene, he said.

He saw traffic in his rearview mirror come to a halt. Soon after, the northbound lanes of the highway crumbled.

"It was crazy timing," Fusetti said. "For it to buckle and collapse that quickly, it's pretty remarkable."

The collapsed section of I-95 was part of a $212 million reconstruction project that wrapped up four years ago, Rudolph said. There was no immediate time frame for reopening the highway, but officials would consider "a fill-in situation or a temporary structure" to accelerate the effort, he said.

Motorists were sent on a 43-mile (69-kilometer) detour, which was going "better than it would do on a weekday," Rudolph said. The fact that the collapse happened on a Sunday helped ease congestion, but he expected traffic "to back up significantly on all the detour areas."

Pennsylvania Transportation Secretary Michael Carroll said the I-95 segment carries roughly 160,000 vehicles per day and was likely the busiest interstate in Pennsylvania. He said work would continue through the night to remove the collapsed section as rapidly as possible.

Shapiro said he had been spoken directly to U.S. Transportation Secretary Pete Buttigieg and had been assured that there would be "absolutely no delay" in getting federal funds quickly to rebuild what he called a "critical roadway" as safely and efficiently as possible.

But Shapiro he said the complete rebuild of I-95 would take "some number of months," and in the meantime officials were looking at "interim solutions to connect both sides of I-95 to get traffic through the area."

White House press secretary Karine Jean-Pierre said in a Twitter post that President Joe Biden was briefed on the collapse and that White House officials were in contact with Shapiro and Philadelphia Mayor Jim Kenney's offices to offer assistance. Buttigieg, in a social media post, called it "a major artery for people and goods" and said the closure would have "significant impacts on the city and region until reconstruction and recovery are complete."

The National Transportation Safety Board said it was sending a team to investigate the fire and collapse.

Most drivers traveling the I-95 corridor between Delaware and New York City use the New Jersey Turnpike rather than the segment of interstate where the collapse occurred. Until 2018, drivers did not have a direct highway connection between I-95 in Pennsylvania and I-95 in New Jersey. They had to use a few miles of surface roads, with traffic lights, to get from one to the other.

Officials were also concerned about the environmental effects of runoff into the nearby Delaware River.

After a sheen was seen in the Delaware River near the collapse site, the Coast Guard deployed a boom to contain the material. Ensign Josh Ledoux said the tanker had a capacity of 8,500 gallons, but the contents did not appear to be spreading into the environment.

"As far as waterways go, it's being contained, and it seems like things are under control," he said.

Thousands of tons of steel and concrete were piled atop the site of the fire, and heavy construction equipment would be required to start to remove the debris, said Dominick Mireles, director of Philadelphia's Office of Emergency Management.

The fire was strikingly similar to another blaze in Philadelphia in March 1996, when an illegal tire dump under I-95 caught fire, melting guard rails and buckling the pavement.

The highway was closed for several weeks, and partial closures lasted for six months. Seven teenagers were charged with arson. The dump's owner was sentenced to seven to 14 years in prison and ordered to pay $3 million of the $6.5 million repair costs, The Philadelphia Inquirer reported.

More recently in Atlanta, an elevated portion of Interstate 85 collapsed in a fire, shutting down the heavily traveled route through the heart of the city in March 2017. A homeless man was accused of starting the blaze. But federal investigators said in a report that the state transportation department's practice of storing combustible construction materials under the highway increased the risk of fire.