By Irene Klotz

CAPE CANAVERAL, Fla., June 22 (Reuters) - Space Exploration Technologies, or SpaceX, postponed for a third time the launch of six commercial communication satellites from Cape Canaveral Air Force Station, officials said on Sunday.

Liftoff of the privately owned company's Falcon 9 rocket had been slated for 5:30 p.m. EDT/2130 GMT. Aboard the rocket are six small satellites owned by Orbcomm Inc, which provides machine-to-machine data and messaging services worldwide.

SpaceX, which is owned and operated by technology entrepreneur Elon Musk, has been trying since Friday to launch what would be its 10th Falcon 9 rocket, a medium-lift booster that also flies cargo capsules to the International Space Station for NASA. SpaceX is pursuing U.S. military launch contracts as well, hoping to break a monopoly by United Launch Alliance, a partnership of Boeing and Lockheed Martin .

Friday's launch attempt was called off due to a potential technical problem with the rocket's upper-stage engine. No other information about the issue was provided by SpaceX, though the glitch apparently was cleared in time for a second launch attempt on Saturday. That attempt was canceled because of poor weather at the launch site.

SpaceX rescheduled a launch for Sunday but encountered another technical issue. It was not known if the glitch was related to the upper-stage engine issue that surfaced on Friday. SpaceX did not immediately respond to a request for comment.

"The ... launch attempt has been scrubbed to address a potential concern identified during pre-flight checks," SpaceX wrote on its website.

"The vehicle and payload are in good condition, and engineering teams will take the extra time to ensure the highest possible level of mission assurance prior to flight," it said.

The next launch opportunity is on Tuesday.

SpaceX said it had planned to restore a webcast and commentary for Sunday's launch attempt after imposing an unprecedented media blackout for Saturday's launch try.

"For the first time since the end of the Cold War, a space launch from Cape Canaveral will not be broadcast to the press and the public," Spaceflightnow.com, which provides live launch coverage, wrote on its website on Saturday.

The blackout spurred an angry backlash on Twitter. The company did not respond to questions about why it canceled Saturday's launch commentary and webcast or why it planned to reverse itself for Sunday's launch attempt.

(Editing by Eric Walsh)