WASHINGTON (Reuters) - U.S. Transportation Secretary Anthony Foxx said on Friday he is concerned about the lack of a major investigation into the downed Malaysian airliner in rebel-held eastern Ukraine.

Asked about the delay so far into probing Thursday's incident, which killed nearly 300 people, Foxx told MSNBC, "It's obviously a matter of concern.

"It's going to require international cooperation to figure out and get to the bottom of what happened," Foxx said. "We stand ready to help."

Investigators from the FBI and the U.S. National Transportation Safety Board personnel are headed to the region to serve in an advisory role for the investigation of Thursday's incident.

Foxx's comments came as world leaders demand an investigation into the crash.

The Malaysian Flight MH17, en route from Amsterdam to Kuala Lumpur, was shot down with 298 people on board. The wreckage is across miles of rebel-held territory near the border with Russia, which is mired in conflict with Ukraine.

Foxx also said U.S. aviation officials expanded the prohibition on U.S. flights over the region near the Ukrainian conflict in the wake of the crash and are continuing to monitor the situation.

"We expect the international community will likely follow our lead," he told MSNBC.

The International Civil Aviation Organization said on Friday it cannot open or close airline routes.

(Reporting by Susan Heavey; Editing by Bill Trott)