(New throughout, adds Malaysian request for undersea surveillance equipment)
By Phil Stewart and David Alexander
WASHINGTON, March 21 (Reuters) - The Pentagon is weighing a request from Malaysia for sonar equipment to bolster the so-far frustrated search for Malaysia Airlines Flight MH370, as concerns grow that any debris may have sunk to the bottom of the sea.
Malaysia's Defense Minister Hishammuddin Hussein asked for undersea surveillance equipment in a phone call with U.S. Defense Secretary Chuck Hagel, as the Pentagon tallied $2.5 million in costs so far in the nearly two-week-old search.
"No specific request was made for any particular type. It was just a general request for us to help them locate the wreckage and/or the black box," Pentagon spokesman Rear Admiral John Kirby told Reuters on Friday.
"The Secretary said he would consider the request, that he would examine whether we had anything that was both available and potentially helpful and that he would get back to the minister in the very near future."
The U.S. Navy has a variety of active and passive sonar systems, some of which search the ocean for objects by emitting sound "pings" and monitoring the echoes that bounce back and others that listen for sound like an undersea microphone.
One system, called a "Towed Pinger Locator", is towed behind ships and is used to listen for downed Navy and commercial aircraft at depths of up to 20,000 feet (6000 meters), according to the U.S. Navy's website.
The U.S. military loaned this technology to France during its two-year effort to locate the black box from an Air France jetliner that crashed into the Atlantic Ocean in June 2009.
The P-8 and P-3 spy planes, which the United States is already deploying in the Malaysian jetliner search, also carry "sonobuoys" that can be dropped into the sea and use sonar signals to search the waters below.
"Sound actually travels a long distance under water, depending on the conditions," Kirby said.
"Temperature, current, the underwater topography, all of these things change the way sound travels underwater. But sound can travel a long, long way."
One big question will be where to drop any sonar equipment.
Investigators suspect Flight MH370, which took off from Kuala Lumpur for Beijing shortly after midnight on March 8, was deliberately diverted thousands of miles from its scheduled path. They say they are focusing on hijacking or sabotage but have not ruled out technical problems.
There has been no confirmed sign of wreckage so far and Australia's Deputy Prime Minister Warren Truss cautioned on Friday that anything once floating "may have slipped to the bottom."
Hishammuddin, who is also acting transport minister, has acknowledged that the clock was ticking.
The plane's "black box" voice and data recorder only transmits an electronic signal for about 30 days before its battery dies, after which it will be far more difficult to locate.
"We've got three more weeks to find those pingers on the black boxes -- or else this plane may never be found," said Alan Diehl, who spent 40 years investigating aircraft accidents for the U.S. National Transportation Safety Board, Federal Aviation Administration and U.S. military.
He said the Pentagon should send submarines and more aircraft.
In its first disclosure of the cost of the U.S. search, the Pentagon estimated about $2.5 million had been spent so far. It added the U.S. Defense Department had set aside about $4 million -- enough to cover operations through early April. (Additional reporting by Andrea Shalal; Editing by Alistair Bell and David Gregorio)