An unmanned Antares rocket exploded seconds after liftoff from a commercial launch pad in Virginia on Tuesday, marking the first accident since Nasa turned to private operators to deliver cargo to the International Space Station.

The 14-story rocket, built and launched by Orbital Sciences Corp, blasted off its seaside launch pad at the Wallops Flight Facility at 6:22 p.m. EDT/2222 GMT, carrying a Cygnus cargo ship bound for the space station. It burst into flames moments later, then plunged to the ground in a huge ball of fire and smoke, but authorities said no one was hurt.

The six crew members in orbit aboard the space station - two Nasa astronauts, one from the European Space Agency and three Russian cosmonauts - watched the launch via a Nasa TV feed, said Mike Suffredini, the space station program manager.

"They were disappointed ... of course they are well aware that they have plenty of resources on orbit," Suffredini told reporters during a conference call.

With a Russian cargo ship due to reach the space station on Wednesday, just 14 hours after the explosion in Virginia, the loss of the Cygnus vessel posed no immediate problem for the orbiting team.

"There was no cargo that was absolutely critical" aboard Cygnus, Nasa Associate Administrator William Gerstenmaier said. Suffredini added that the crew has enough food and other supplies aboard to last four to six months.

Orbital Sciences stock fell 15.5 per cent to a two-month low of $25.65 in after-hours trading.

The cause of the mishap was under investigation, said Frank Culbertson, Orbital Sciences executive vice president.

Footage of Tuesday's launch showed the Antares rising slowly into the night sky as flames suddenly engulfed the rocket, from the bottom to the top about 11 seconds after liftoff, before the vehicle sank back downward in a conflagration.

Ronda Miller, manager of the Ocean Deli in Wallops Island, Virginia, told Reuters she felt the force of the blast from the eatery, about 5 miles (8 km) from the launch pad.

'IT SHOOK THE WHOLE BUILDING'

"We were standing outside waiting for it to launch and we saw bright red, and then we saw a big black cloud, and it shook the whole building where we work," Miller said. "And then I came back in to work and seen fire trucks going every which way."

Nasa said no one was in the vicinity of the explosion, and Orbital Sciences said in a statement: "We've confirmed that all personnel have been accounted for. We have no injuries in the operation today."

The spacecraft was carrying "some classified cryptographic equipment, so we do need to maintain the area around the debris in a secure manner," said Mike Pinkston, the company's Antares program manager.

The National Security Agency had no immediate comment on the classified equipment reported to be aboard the cargo ship.

Nasa officials said damage on the ground appeared limited to the Virginia-owned launch facility, but its full extent was not immediately known. The rocket itself and the cargo ship it carried were valued at $200 million, Culbertson said.

"This is the only pad that's certified for launching the Antares rocket, so repairing it will be one of our high priorities," he told reporters in a conference call. "We will not fly until we understand the root cause and the corrective action to make sure this doesn't happen again."

The Antares rocket has been launched successfully on four previous missions. Tuesday's launch had been delayed one day after a boat sailed into a restricted safety zone beneath the rocket's intended flight path.

Virginia-based Orbital Sciences is one of two companies hired by Nasa to fly cargo to the station after the space shuttles were retired. Tuesday's planned flight was to be the third of eight under the company's $1.9 billion contract with Nasa.

SPACEX LAUNCH SET FOR DECEMBER

The second U.S. supply line to the station is run by privately owned Space Exploration Technologies, or SpaceX, which is preparing for its fourth flight under a separate $1.6 billion Nasa contract, with launch slated for Dec. 9.

Outfitted with a new, more powerful upper-stage engine, the Antares rocket launched on Tuesday carried a Cygnus spacecraft packed with 5,055 pounds (2,293 kg) of supplies, science experiments and equipment, a 15 per cent increase over previous missions.

The Cygnus capsule was the first of two cargo ships scheduled to head to the space station this week. Russia was preparing to launch a Soyuz rocket and Progress freighter from the Baikonur Cosmodrome in Kazakhstan on Wednesday morning.

Cygnus was to loiter in orbit until Nov. 2, then fly to the station so astronauts can use a robotic crane to snare the capsule and attach it to a berthing port. The station, a $100 billion research laboratory owned and operated by 15 nations, flies about 260 miles (418 km) above Earth.

In addition to food, supplies and equipment, the Cygnus spacecraft was loaded with more than 1,600 pounds (725 kg) of science experiments, including an investigation to chemically analyze meteors as they burn up in Earth's atmosphere.

The Cygnus also carried a prototype satellite owned by Redmond, Washington-based startup Planetary Resources Inc., which is developing technology to mine asteroids. The satellite, designated A3, was to be released into space by a commercially owned small spacecraft launcher aboard the station.

Orbital Sciences is in the midst of merging with Alliant Techsystem Inc's Aerospace and Defense division, a deal that analysts expect to close sometime early next year. The company is competing for a number of Nasa missions or projects that will help fuel revenue over coming years. – Reuters