An investigation was under way on Monday into why a tanker fire collapsed a section of the US east coast’s main north-south road, throwing hundreds of thousands of morning commutes into chaos and disrupting commerce for untold numbers of businesses.

Interstate 95 in Philadelphia, Pennsylvania, will be closed in both directions for weeks at the start of the summer travel season.

Motorists should expect extensive delays and street closures, and avoid the north-east corner of the sixth-largest city in the country, transport officials said.

The accident also disrupted the car route from Canada to Florida through the Boston, New York and Washington metropolitan areas.

Pennsylvania transportation secretary Michael Carroll said the I-95 segment carries roughly 160,000 vehicles per day.

Sunday’s fire sent plumes of black smoke into the air. The north-bound lanes collapsed and the south-bound counterparts were compromised, according to the Philadelphia fire department.

Authorities have not publicly identified the lorry owner or the driver, indicated whether the driver survived, or said what exactly fuelled the blaze.

Governor Josh Shapiro signed a disaster declaration on Monday, saying it gives state agencies the ability to skip normal bidding and contracting requirements so that the road can be repaired more quickly.

Mr Shapiro said Sunday that no motorists on the highway were injured or died, although videos shared on social media showed some close calls, with people driving through as flames licked upward from the fire below.

Officials said the tanker contained what may have been hundreds of gallons of petrol. The fire took about an hour to get under control.

High heat from the fire or the impact of an explosion could have weakened the steel beams supporting the overpass, according to Drexel University structural engineering Professor Abi Aghayere.

Bridges like the one that collapsed do not typically have fire protection, like concrete casing, he added. It could have been coated in a fire-retarding paint, but even then the beams could have been weakened.

“It just gives you time,” he said.

The collapsed section of I-95 was part of a 212 million dollar (£170 million) reconstruction project that finished four years ago, state transportation department spokesman Brad Rudolph said.

The 104ft elevated section was in “good” condition earlier this year, with another inspection set for 2025.