An unmanned rocket exploded in a fireball just after it launched from a NASA launchpad in Virginia on Tuesday evening, ending a planned supply mission to the International Space Station moments after it launched in what scientists called a "catastrophic anomaly."

Scientists were scrambling to figure out what went wrong as Orbital Sciences' Antares rocket, carrying an unmanned Cygnus capsule hauling thousands of pounds of space gear, exploded just six seconds after its 6:22 p.m. ET blast-off from the Wallops Flight Facility.

Flames shot and smoke billowed into the sky when the spacecraft blew up as the sun went down over Wallops Island on Virginia's eastern shore.

No injuries were reported in the first catastrophic launch yet in NASA's commercial spaceflight program, and all personnel were accounted for, the launch director said soon after the blast.

There was significant damage to the facilities, however. The Cygnus capsule was carrying "classified cryptoequipment," and the area around the debris field needed to be secured for accident investigation and security reasons, the launch director said. People were warned not to approach debris.

"Maintain your consoles," Orbital Sciences' Mission Control informed the roomful of engineers and technicians. All data were being collected to use in the ensuing investigation.

Orbital's executive vice president Frank Culbertson promised a thorough probe of the cause of the failure and a return to flight thereafter in a statement released Tuesday night.

"It is far too early to know the details of what happened. As we begin to gather information, our primary concern lies with the ongoing safety and security of those involved in our response and recovery operations," he said.

Another official said on NASA's livestream of the explosion's aftermath that the countdown had been "flawless" and that the launch team had not been tracking any known issues.

The Cygnus cargo ship was loaded with 5,000 pounds of experiments and equipment for the six people living on the space station. It was the fourth Cygnus bound for the orbiting lab; the first flew just over a year ago.

NASA spokesman Rob Navias said there was nothing urgently needed by the space station crew on that flight. In fact, the Russian Space Agency was proceeding with its own supply run on Wednesday. All the equipment lost was replaceable, officials said at an evening press conference.

Data from the failure was already being analyzed, and investigators were headed to the scene Wednesday, officials added.

The National Transportation Safety Board and the Federal Aviation Administration were expected to perform the first investigation.

The launch had been supposed to kick off the third in a series of eight planned Orbital delivery missions to astronauts.

NASA is paying the Virginia-based Orbital Sciences and the California-based SpaceX company to keep the space station stocked in the post-shuttle era. This is the first disaster in that effort.