A Trident II ballistic missile launched from a US submarine. U.S. Navy China is finding itself falling into a strategic nightmare with the first sign of a Washington-Tokyo­-Seoul military alliance at its ­doorstep after South Korea hinted that it would share missile intelligence with Japan, analysts say.

South Korea's Ministry of ­Defense said only that it could share with Japan the information on North Korean missiles gathered via a US-supplied ­antimissile system. But that is a dangerous step in the eyes of ­Beijing, as it could knit Tokyo and Seoul closer in military cooperation down the road.

Both Japan and South Korea are military allies of the US, but Seoul is always reluctant to engage in bilateral military cooperation with Tokyo because of territorial disputes and wartime atrocities suffered by Koreans.

But Seoul's stance changed on Thursday.

In his regular press conference, the South Korean defense ministry spokesman said information sharing with Japan would be possible, citing a memorandum signed in 2014 by the US, South Korea, and Japan regarding Pyongyang's nuclear and missile programs, Japan's Kyodo News Agency reported.

This modest start could lead to wider information-sharing ­between South Korea and Japan, and remotely, a military alliance, said Song Zhongping, a Beijing-based military analyst who previously worked as an instructor for the People's Liberation Army's Second Artillery Corps, the former strategic missile force.

"This could mean a three-party alliance, rather than two-sided alliances [of the US and Japan, and the US and South Korea], and this would pose a damaging threat to the stability of Northeast Asia," Song said.

If South Korea drifts into the orbit of the US and Japan, China's influence on the Korean peninsula could be badly compromised.

via Wikimedia

At a military parade in Beijing to mark the 70th anniversary of the surrender of Japan last ­September, South Korean President Park Geun-hye was the only American ally present, standing with President Xi Jinping and Russian President Vladimir Putin.

A few months later, the US and Seoul announced they would ­deploy the Terminal High Altitude Area Defense, or THAAD, system. Its stated purpose is to counter North Korea's missile threat, but it could also be used to watch China.

China was so infuriated that Beijing told its television stations to suspend any new shows with South Korean stars, sources said earlier this week.

Xu Guangyu, a retired PLA ­major general, said China would be pushed into a corner if South Korea and Japan widened their collusion, giving China's leaders no choice but to lean toward a Beijing-Moscow alliance to ­provide a counterbalance.

"In such a case, China and Russia would face a powerful challenge from the US, South Korea, and Japan, who can obtain missile information about China and Russia in a short time and take immediate action," Xu said.

"This would in turn trigger a stronger backlash from China and Russia and lead to an arms race in Northeast Asia."