SINGAPORE, Oct 20 - Several countries including the United States and China agreed "in principle" on Saturday to multilateral guidelines to manage unexpected encounters between their military aircraft, joining 10 Southeast Asian nations already in the pact.The world's two biggest economies as well as Australia, India, Japan, New Zealand, Russia, South Korea tentatively joined the agreement, which was initially adopted on Friday by the Association of Southeast Asian Nations (ASEAN), according to a joint statement issued after a meeting of defense ministers from the 18 countries in Singapore.The voluntary, non-binding guidelines build on an existing code to manage sea encounters adopted by all 18 countries last year, which was designed to mitigate risks following a boom in the region's maritime and air traffic in recent years."We all know that if there is a physical incident it changes the name of the game...it creates a cascade of activities that you cannot control," Singapore defense minister Ng. Eng Hen, the host, said at a press briefing following the meeting.The air code has been hailed as the first multilateral deal of its kind, although such arrangements exist at bilateral levels. The U.S. and China, for instance, in 2015 signed a pact on a military hotline and rules governing air-to-air encounters.U.S. Defense Secretary Jim Mattis told his Chinese counterpart, Wei Fenghe, on Thursday that their countries needed to deepen high-level ties so as to navigate tension and rein in the risk of inadvertent conflict.The U.S. military flew B-52 bombers across the South China Sea in September. Earlier this month, a U.S. Navy destroyer sailed near islands China claims, drawing the ire of Beijing.