* Plane left from U.S. transit centre in Kyrgyzstan - ministry
* Base used to transport troops in and out of Afghanistan
* Rescuers on horseback searching mountains for crew
* Civil aviation official says storm clouds could be cause
A U.S. military refuelling plane on its way to Afghanistan exploded in mid air and crashed in Kyrgyzstan on Friday when its cargo of fuel ignited, the Central Asian country's Emergencies Ministry said.
The aircraft took off from the U.S. military transit centre at Kyrgyzstan's international Manas airport, which U.S. forces maintain for operations in Afghanistan, with around 70 tonnes of fuel on board, a local ministry official said.
The plane, used for inflight refuelling, disappeared from radar screens at 3:10 p.m. as it flew near the Kyrgyz village of Chaldovar, the ministry said. The three person crew was unaccounted for, it said.
The ministry said witnesses saw the plane explode in the air, and a local government official said debris was scattered over a 4 to 5 km area in a nearby mountainous area.
"The chassis, the fuselage have all been extinguished and now we are searching for the crew members and the plane's black box," ministry official Bolot Sharshenaliyev said.
"There are three crew members. According to preliminary information it flew from Bishkek and was on its way to Afghanistan, with approximately 70 tonnes of fuel on board."
Cellphone video footage obtained by Reuters showed an armed guard around the crash site where burning debris had scorched the ground.
Rescue workers on horseback were scouring the area before night fell in the rural area on the border with Kazakhstan, regional governmental official Kanat Davletov told Reuters.
The U.S. transit centre confirmed the crash of the Boeing KC-135 Stratotanker and said it was under investigation. The centre did not give a cause for the crash nor confirm the number of crew members on board.
A Kyrgyz civil aviation official, speaking on condition of anonymity, said storm clouds over the region could have caused the explosion, Interfax reported.
Manas, established as a military base in December 2001, is used to transport U.S. personnel in and out of Afghanistan and has been used by other international forces in the country.
U.S. forces lease the facility for $60 million a year.
The contract for the transit centre, near the Kyrgyz capital of Bishkek, will expire in July of next year, as Washington draws down its forces from Afghanistan.