SpaceX said it was "optimistic" Friday after a thruster outage delayed the latest resupply mission of its unmanned Dragon capsule en route to the International Space Station.

SpaceX and NASA officials said the cargo resupply mission was still on track, but the technical mishap could fuel concerns about the US agency's ambitious plans to cut costs by privatizing elements of the space program.

SpaceX's billionaire founder Elon Musk said the failure of three out of four thruster pods to fire up was a "little frightening" but that two pods were back online within a few hours and the others should be working again shortly.

"I'm optimistic that we will be able to turn all four thruster pods on and restore full control," he told reporters.

Musk later tweeted: "Thruster pods one through four are now operating nominally. Preparing to raise orbit. All systems green."

SpaceX and NASA officials said once the pods are back online, they would carry out a number of checks before clearing the vessel to dock at the space station in the coming days, perhaps as early as Sunday.

The original rendezvous had been planned for 1130 GMT Saturday, but Mike Suffredini, NASA program manager for the International Space Station, said there was "quite a bit of flexibility" in the berthing date.

He also praised the handling of the mishap, saying "it was a pleasure to watch the SpaceX team in action" as they worked through "anomalies."

The malfunction occurred shortly after the capsule achieved orbit and separated from the Falcon 9 rocket after a near-perfect launch earlier in the day from Cape Canaveral, Florida.

Musk said it was unclear what caused the problem, but suggested a valve leading to the oxidation tanks may have been blocked.

"I don't think it's a major concern. This is the first time we've seen this type of issue," he said. "I think it's an anomaly."

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 flight.

The first resupply mission in 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.

"This unique vehicle has become a very integral part of how we operate and use the space station," Suffredini said on Thursday, as he described plans for the 25-day mission.

The cargo includes equipment for 160 experiments to be conducted by the space station crew, which currently consists of two Americans, three Russians and a Canadian.

On the return flight, Dragon -- the only spacecraft able to bring cargo back to Earth for now -- will be loaded with just over a ton of materials, including results of medical research.

Before the thruster outage, the capsule had been scheduled for a splashdown landing off the coast of California on March 25.

NASA has bet on SpaceX and other commercial ventures to take over for its retired fleet of space shuttles, which last flew in July 2011.

Before SpaceX's successful mission in October, NASA had been relying on Russian spacecraft -- but the Soyuz craft does not have room for cargo on the return flight.

SpaceX says it has 50 launches planned -- both NASA missions and commercial flights -- representing about $4 billion in contracts.

So far, SpaceX has only sent unmanned flights into orbit, but the company aims to send a manned flight within the next three or four years. It is under a separate contract with NASA to refine the capsule so that it can carry a crew.

NASA also has a $1.9 billion resupply contract for the station with Orbital Sciences Corporation, which will launch the first test flight of its Antares rocket from a base in Virginia in the coming weeks.