It has become a formulated remark by Seoul officials that South Korea will continue to strengthen its security alliance with the U.S. while expanding its strategic partnership with China. But they find it increasingly tough to strike this delicate balance between the two superpowers at a growing rivalry with expanded areas of conflict.

A concrete case showing the difficulty of the complicated task is Washington’s persistent pressure on Seoul to join the U.S.-led regional missile defense shield.

In the latest move to this end, the U.S. House of Representatives last week passed a bill calling on the Pentagon to explore ways to boost ballistic missile defense cooperation with Seoul and Tokyo. If the 2015 National Defense Authorization Act is enacted after being approved by the Senate, the defense secretary should conduct an assessment to identify opportunities to increase the three-way cooperation and submit a related report to Congress within six months of its effectuation.

Seoul has remained reluctant to join the U.S.-led regional missile defense program out of concern that its participation would cause friction with China and Russia, which see it as targeting them. China, in particular, will react sensitively to the strengthening of the trilateral security cooperation as a move to keep its rising power in check.

South Korea has sought to deploy an independent missile shield, called the Korea Air and Missile Defense system, which is designed to destroy incoming missiles, presumably from North Korea, at low altitudes.

On the surface, Seoul and Washington have managed to bridge their differences by emphasizing the need to enhance the interoperability between the two systems. President Park Geun-hye and her U.S. counterpart Barack Obama followed this rhetoric on the matter at a news conference following their talks in Seoul last month.

As suggested by the recent U.S. Congressional move, however, Washington is seen to be ratcheting up pressure on Seoul to get closer to it than to Beijing on the missile defense issue. But South Korea will continue to find it hard to accept the U.S.’ request, especially as Chinese President Xi Jinping’s visit to Seoul, which is expected to come in June, will further boost the bilateral partnership.

Seoul’s realistic option, though much easier said than done, seems to maximize the interoperability of the two missile defense schemes without provoking official protest from Beijing. China’s more efficient role in reining in North Korea’s belligerence will help Seoul make this case.