* Wary regional rivals set aside differences to hunt for MH370
* Chinese, Japanese, Australian search crews share lunch
* Close watch on capability, especially U.S. Navy P8
PERTH/SYDNEY, Australia, March 28 (Reuters) - The pot-bellied silhouette of a Chinese Il-76 military transport plane appeared in the sky over Perth International Airport just as the U.S. naval officer was explaining how he guards his cutting-edge surveillance plane.
Lieutenant Commander Adam Schantz was ticking off the measures, including a round-the-clock guard and armed rapid response team, as he caught sight of the Chinese aircraft coming in to land a few metres from the U.S. P8 Poseidon for which he is responsible.
The search for missing Malaysia Airlines flight MH370 is producing strange bedfellows.
"Yeah, it's a little different," Schantz said with a laugh.
At least six countries - the United States, China, South Korea, Japan, New Zealand and Australia - are participating in the search and rescue operation for the flight, which disappeared almost three weeks ago and is believed to have crashed in the southern Indian Ocean off Australia's west coast.
The level of military cooperation between a grouping of countries that contains several traditional antagonists has been unprecedented. But as the wary allies focus on solving this mystery, they are keenly aware of the boundaries of cooperation, diplomatic or military.
"When they are out there and the U.S. is using its sensors, you can be absolutely sure that the Chinese are recording all of that and are analysing how it's done because that's very useful in understanding how the P8s work," David Brewster, a visiting fellow at the Strategic Defence and Studies Centre at the Australian National University, told Reuters.
The Poseidon, an anti-submarine warfare and electronic signals interception plane manufactured by Boeing Co, is the most advanced of its type. Australia, New Zealand, Japan and South Korea all operate an earlier model, the P3 Orion, while China has the larger Russian-made Ilyushin.
The P8 only entered service in 2013 and information on its sophisticated sensors could be a prime target for Chinese intelligence.
"I'm not surprised to see a lot of security. There's a lot of political sensitivity," Brewster said.
Western forces are also keeping their eyes open. Air search crews told Reuters that Australian personnel were flying with the Chinese on their sorties. It was not clear if that was for security reasons or to assist with communications after the first Chinese aircraft to fly into Perth landed at the wrong airport last weekend.
Chinese Defence Ministry spokesman Geng Yansheng said on Thursday that Chinese forces had had "effective communication" with the other countries taking part in the search, including "close coordination" with the Australians and Malaysians.
Then there are diplomatic sensitivities, chief among them the stormy relationships between Japan and its historical foes, regional superpower China and South Korea. By all accounts that has gone more smoothly than many had expected.
Earlier this week, Australian Defence Minister David Johnston hosted a lunch at Perth's RAAF Base Pearce mess hall with a dozen representatives of both the Chinese and Japanese search teams, as well as the Australians.
A defence ministry source in attendance told Reuters the teams enjoyed corned beef or chicken noodles in a convivial atmosphere, albeit sitting in their national groupings.
"It was very amiable and relaxed, there was no tension whatsoever, it was all friendly - all very professional," the source told Reuters under condition of anonymity because the person was unauthorised to speak to the media.
Bringing together countries like China and Japan, which have a history of tensions over strategic grievances going back beyond World War Two, does raise cooperation issues, said Andrew Davies, a senior analyst for defence capability at the Australian Strategic Policy Institute.
But bigger political realities often faded, he said, when the issue at hand was getting the job done.
"You might be surprised at how down-to-business the actual operators can be on the ground," he said.
(Additional reporting by Lincoln Feast in Sydney and Michael Martina in Perth and Ben Blanchard in Beijing; Editing by Ron Popeski)