Ten ships and as many aircraft are searching a huge area in the southern Indian Ocean west of Perth, trying to find some trace of the aircraft, which went missing more than three weeks ago.
Numerous objects have been spotted in the two days since Australian authorities moved the search 685 miles after fresh analysis of radar and satellite data concluded the Boeing 777 had travelled faster over a shorter distance after vanishing on March 8.
US Navy Captain Mark Matthews said: "Right now the search area is basically the size of the Indian Ocean, which would take an untenable amount of time to search."
In reference to a plane that crashed in 2009 near Brazil and took more than two years to find, he added: "If you compare this to Air France flight 447, we had much better positional information of where that aircraft went into the water."
The US Navy cannot use sonar location devices, which can listen for beacons on the aircraft's flight data and cockpit voice recorders, until "conclusive visual evidence" of debris is found.
If no location is found, searchers would have to use sonar to slowly and methodically map the bottom of the ocean.
US Navy spokesman Commander William Marks said: "That is an incredibly long process to go through. It is possible, but it could take quite a while."
Among the vessels to join the search is an Australian defence force ship, the Ocean Shield, which has been fitted with a black box locator and an underwater drone.
Malaysia says the plane, which disappeared less than an hour into a flight from Kuala Lumpur to Beijing, was likely diverted, but investigators have determined no motive or other red flags among the 227 passengers or 12 crew.
The search has involved unprecedented cooperation between more than 24 countries and 60 aircraft and ships but has also been hampered by national rivalries and reluctance to share potentially crucial information due to security concerns.
Asked if more resources could be added, US Navy spokesman Marks said: "We have about as many assets out there as we can. You have to wonder if the debris is even out there. If we fly over something, we will see it."
Australia issued new protocol to all parties in the search, giving Malaysia authority over the investigation of any debris to be conducted on Australian soil.
A spokeswoman at the Department of Foreign Affairs and Trade said: "Australia intends to bring the wreckage ashore at Perth and hold it securely for the purposes of the Malaysian investigation."
Australian Prime Minister Tony Abbott yesterday appointed Air Chief Marshal Angus Houston to head a new Joint Agency Coordination Centre.
The JACC will coordinate communication between all search partners as well as with the families of passengers, many of whom are expected to travel to Perth.
The Malaysian government has come under strong criticism from China, home to more than 150 of the passengers, where relatives of the missing have accused the government of "delays and deception".
Dozens of angry relatives of Chinese passengers from Beijing met with Chinese embassy officials in Kuala Lumpur, piling more pressure on the Malaysian government over its handling of the case.
One victim relative, Jiang Hui, said: "We want the Malaysian government to apologise for giving out confusing information in the past week, which caused the delay in the search and rescue effort."