By Matt Siegel
SYDNEY, April 25 (Reuters) - The search for missing Malaysia Airlines Flight MH370 is likely to drag on for years, a senior U.S. defence official told Reuters on Friday, as an underwater search for any trace of the plane's wreckage off west Australia appeared to have failed.
The official, speaking under condition of anonymity because he was not authorised to comment on the search effort, said two weeks of scouring the Indian Ocean floor with a U.S. Navy submersible drone had turned up no wreckage.
He said the search for the jetliner, which vanished on March 8 with 239 people on board, would now enter a much harder phase of scouring broader areas of the ocean near where the plane is believed to have crashed.
"We went all in on this small area and didn't find anything. Now you've got to go back to the big area," the official said. "And now you're talking years."
On Friday, the undersea drone Bluefin-21 is expected to finish what may be the last of its 16-hour trips to depths of more than 4.5km (2.8 miles) searching a 10 square km (6.2 square mile) stretch of seabed about 2,000 miles northwest of Perth.
Authorities had identified the area as their strongest lead in determining the plane's final resting place after detecting what they suspected was a signal, or "ping", from the plane's black box recorder on April 4.
But the U.S. official said Malaysia would have to decide how to proceed with the search, including whether to bring in more underwater drones, even with the understanding that the search could continue for years without a refined search area. (Reporting by Matt Siegel; Editing by Paul Tait)