ABOARD A VIETNAM AIR FORCE AN- (Reuters) - Ships from six navies, dozens of military aircraft, sweeps with radar technology that can spot a soccer ball from hundreds of feet in the air - all have failed to find a single confirmed trace of a Malaysia Airlines plane that vanished three days ago.
For the sailors and aircrew, it has been a frustrating operation. One reported sighting of the plane's tail turned out to be a few logs tied together.
On Monday, a Vietnamese jet spotted what it believed was a yellow airline life raft floating in the sea. A helicopter scrambled to investigate pulled a moss-covered cable reel cover from the sea.
The massive search is mainly in a 50-nautical mile radius from the point of last radar contact with the plane, midway between Malaysia's east coast and the southern tip of Vietnam. It's an area of about 27,000 sq km (10,500 sq miles) that includes parts of the Gulf of Thailand and the South China Sea.
There was little commercial shipping in the area on Monday when a Reuters photographer boarded a Vietnamese air force Antonov-26, a Soviet-era transporter, on an aerial sweep.
The plane spent two-and-a-half hours circling a 400 sq km (155 sq mile) area at about 6,500 ft (1,980 metres), but found nothing as crew members peered down with binoculars.
"The main challenge that we face during these searches is that we have to search with bare eyes from a high altitude," said Captain Vu Duc Long of Vietnam's 918 Squadron.
"We sent out three planes this morning, two conducting the search and the other being the commanding craft. But as of right now, we haven't found anything."
No confirmed sighting of any debris or part of the plane has been reported since flight MH370 vanished from radar screens early on Saturday morning, about an hour into a flight from Kuala Lumpur to Beijing.
Scores of potential sightings have been checked out.
Typically, aircraft report sightings of objects in the water, if possible with photographs, to Malaysian authorities who are coordinating the search. The nearest low-flying aircraft, helicopter or ships are sent to verify the reports.
Malaysia has expanded the search to its west coast after theories that the plane may have turned back toward Kuala Lumpur for some reason. A total of 34 aircraft and 40 ships from 10 nations are involved in the search.
The U.S. 7th Fleet has sent a P-3C Orion surveillance plane from its base in Okinawa, Japan, and the USS Pinckney destroyer that is equipped with two MH-60R Seahawk helicopters for search and rescue.
The Orion was used for more than three hours on Sunday, sweeping about 4,000 sq km every hour. It is equipped with the APS-147, an advanced radar system that can identify a soccer ball bobbing in the water from hundreds of feet in the air.
The Seahawks have been used for night searches, using a forward-looking infra-red camera.
"There are lots of challenges," said Commander William Marks, a spokesman for the 7th Fleet.
"First should the central point be the point of last communication or last radar contact? Then you have to account for winds and currents. Every hour, the area gets bigger. It's been three days since the plane was reported missing, it's a very large area."
China has sent four naval ships, a coastguard vessel and a civilian ship to help. Three other Orions have also been deployed - two from Australia and one from New Zealand.
Besides militaries and hi-technology, many others are involved in the search.
"We've ordered border guard forces and all fishing boats to check the area," Pham Thanh Tuoi, chairman of the People's Committee of Vietnam's southern Ca Mau Province, told Reuters by phone. "Everyone is on the alert and searching out at the sea, but we haven't found anything yet."
(Additional reporting by Siva Govindasamy and Raju Gopalakrishnan in Kuala Lumpur, Mai Nguyen in Hanoi, Nguyen Phuong Linh on Phu Quoc Island and Nguyen Ha Minh in Ho Chi Minh City; Writing by Raju Gopalakrishnan; Editing by Alex Richardson)