* Rocket burst into flames soon after blasting off

* Explosion poses no problem for orbiting six-member crew

* Second resupply vehicle launches in Kazakhstan

* Russian space agency offers help to ferry extra US cargo

By Irene Klotz

CAPE CANAVERAL, Florida, Oct 29 (Reuters) - An unmanned U.S. supply rocket exploded shortly after lifting off from a commercial launch pad in Virginia, the first disaster since NASA turned to private operators to run cargo to the International Space Station.

The 14-story Antares rocket, built and launched by Orbital Sciences Corp, blasted off from the Wallops Flight Facility at 2222 GMT on Tuesday but burst into flames moments later and plunged back to the ground in a massive ball of fire and smoke.

No one was hurt in the crash, authorities said. The craft was carrying a Cygnus cargo ship bound for the station, a $100 billion research laboratory owned and operated by 15 nations that orbits about 260 miles (418 km) above Earth.

Another resupply vehicle, the unmanned Russian Progress spacecraft, successfully launched from the Baikonur Cosmodrome in Kazakhstan just hours later with nearly 3 tons of food, fuel and supplies. That craft was due to reach the station later on Wednesday.

The loss of the Cygnus supply vessel posed no immediate problem for the orbiting team of six crew members - two NASA astronauts, one from the European Space Agency and three Russian cosmonauts - officials said.

"There was no cargo that was absolutely critical to us that was lost on that flight. The crew is in no danger," NASA Associate Administrator William Gerstenmaier said.

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

Russia's Roskomos space agency said it was ready to help ferry extra U.S. cargo to the International Space Station if NASA requested such assistance.

"If we get a request to urgently deliver some U.S. cargo to the ISS with the help of our freighter, we will satisfy it," RIA news agency quoted a senior Roskosmos official, Alexei Krasnov, as saying. "But there has been no such request for now."

Cygnus had been due to loiter in orbit until Nov. 2, then fly to the station so astronauts could 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, orbits about 260 miles (418 km) above Earth. It is overseen by Russia and the United States, whose relations are at a low ebb over the Ukraine crisis.

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

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.

"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.

Orbital Sciences stock fell 15.5 percent to a two-month low of $25.65 in after-hours trade.

The Cygnus mission was non-military, but the company's Antares program manager, Mike Pinkston, said the craft included "some classified cryptographic equipment, so we do need to maintain the area around the debris in a secure manner."

RUSSIAN ROCKET ENGINES

The accident renewed questions about the use of Russian engines in U.S. rockets.

The Antares is powered by the AJ-26 engine built by GenCorp Inc division Aerojet Rocketdyne. In May, an AJ-26 exploded during a ground test at NASA's Stennis Space Center in Mississippi.

Orbital Sciences and Aerojet have not yet released the cause of that engine failure.

Congress has been more concerned about Russian-made RD-180 engines that power United Launch Alliance's Atlas 5 rockets, used primarily to fly U.S. military satellites.

The RD-180 has had no technical problems, but Russia has threatened to suspend exports in response to U.S. trade sanctions prompted by Moscow's annexation of Ukraine's Crimea region. United Launch Alliance is a partnership of Lockheed Martin and Boeing.

It was unclear how much Tuesday's explosion would cost Orbital Sciences, whose flight was partly insured. The rocket itself and the cargo ship it carried were valued at $200 million, Culbertson said.

NASA officials said damage on the ground appeared limited to the launch facility, but its full extent was not immediately known.

Culbertson told reporters on a conference call that the pad was the only one certified for launching the Antares rocket, so its repair was a top priority, adding: "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 prior missions. Tuesday's launch attempt had been delayed a 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 NASA has hired 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.

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, slated for Dec. 9.

The Cygnus carried a prototype satellite owned by Redmond, Washington-based startup Planetary Resources Inc., which is developing technology to mine asteroids. The company said it still planned to have a second vehicle included on a U.S. commercial launch in 2015.

Orbital Sciences is in the midst of merging with Alliant Techsystems Inc's Aerospace and Defense division, a deal that analysts expect to close sometime early next year. (Writing by Curtis Skinner; Editing by Gareth Jones)