(Updates with northbound lanes open, other details)
By Dan Whitcomb
LOS ANGELES, May 6 (Reuters) - Northbound lanes of the major freeway connecting Southern California and Las Vegas reopened on Tuesday, more than a day after a bridge fire and collapse forced officials to close the busy highway in both directions.
But the southbound side of Interstate-15 remained closed near the high desert community of Hesperia, about 70 miles (113 km) northeast of Los Angeles, backing up traffic for miles as crews worked to clear tons of mangled metal and debris from the roadway.
An interchange had been under construction over the four-lane highway when it burst into flames on Monday afternoon, officials say, possibly from a worker using a blow-torch to weld part of the metal structure.
No injuries were reported but the fire and subsequent collapse of the bridge forced authorities to close the interstate in both directions, cutting off the main artery between Southern California and Las Vegas, as well as other destinations in Nevada and Utah.
Both northbound lanes of the freeway were reopened shortly after 5 p.m., said San Bernardino Associated Governments spokesman Tim Watkins.
"Crews are already working on the clean-up of the southbound lanes in an effort to open that direction by tomorrow's morning commute," Watkins said.
In the meantime, southbound cars and trucks were being diverted around the closed section of highway using on- and off-ramps, a process that created a miles-long back-up on the desert highway.
Los Angeles TV stations showed long lines of cars and interviewed frustrated drivers, although others took the situation in stride.
"Look where you're at, you're in Southern California. If you live in L.A., you're in traffic 24 hours a day anyway," driver Mike Ingram told local KABC-TV.
The interchange being built over Interstate-15 was part of a $59 million project that began in January of 2013 and was scheduled for completion early next year. Watkins said it was not yet clear how the collapse would affect that schedule.
According to the Las Vegas Convention and Visitors Authority, more than one in four people who visit Las Vegas come from Southern California, with an average of 42,000 cars crossing the California-Nevada border on the I-15 each day.
There are few alternatives for drivers traveling across the California desert between the two destinations. (Reporting by Dan Whitcomb; Editing by Cynthia Johnston, James Dalgleish and Eric Walsh)