The unmanned Dragon capsule launched by US private firm SpaceX encountered problems en route to the International Space Station on Friday when three of its four thrusters malfunctioned, the company said.

"Issue with Dragon thruster pods. System inhibiting three of four from initializing. About to command inhibit override," the company's billionaire founder Elon Musk wrote on the micro-blogging website Twitter.

SpaceX spokeswoman Christina Ra said the spacecraft "experienced an issue with a propellant valve" after reaching orbit.

"One thruster pod is running. We are trying to bring up the remaining three. We did go ahead and get the solar arrays deployed. Once we get at least two pods running, we will begin a series of burns to get to station," she said.

The malfunction occurred shortly after the capsule achieved orbit and separated from the Falcon 9 rocket that launched it earlier in the day from Cape Canaveral, Florida.

The Dragon vehicle is carrying 1,200 pounds (544 kilograms) of supplies on the firm's second resupply mission to the ISS. The two missions were preceded by a nearly flawless test mission.

The first resupply mission last October was a milestone in US efforts to cut costs by privatizing space exploration. The current mission is the second of 12 planned trips in NASA's $1.6 billion contract with SpaceX.