South Korea now faces the possibility of being driven into making another thorny choice between the U.S. and China, with its effort to maintain strategic ambiguity between the two superpowers seeming to go nowhere.

A senior U.S. official urged South Korea last week to speak out against China’s assertion of its territorial claims in the South China Sea. During a seminar in Washington, Assistant Secretary of State Daniel Russel denounced the land reclamation projects that China has been carrying out in an apparent bid to bolster its territorial claims in the waters also claimed by other countries like the Philippines and Vietnam.

The U.S. has openly criticized China for raising tensions with neighboring nations by creating the artificial islands and military facilities in the sea. Washington has vowed to continue air reconnaissance over the area despite protests from Beijing, calling on its allies to join its denunciation of China’s move, which it says destabilizes the international order.

Russel said the fact that South Korea is not a claimant in the disputed waters gives Seoul all the more reason to speak out, because it is speaking not in self-interest but in support of universal principles and the rule of law.

His remarks marked the first time for a U.S. official to make a public request that Seoul take the side of Washington over an issue with the risk of being flashed into a military conflict between the two great powers.

A joint statement issued after last year’s meeting of foreign and defense ministers between South Korea and the U.S. noted they agreed on the importance of maintaining peace and stability and protecting the freedom of navigation in the South China Sea. This can be regarded as indicating that Seoul’s position is different from Beijing’s policy to establish its sovereignty over nearly all of the disputed waters.

But Russel pushed South Korea to go further to clarify its stance against China’s expansionist move. Behind his request seems to be Washington’s concern that Seoul is being tilted toward Beijing.

South Korea decided in March to join the China-led Asian Infrastructure Investment Bank, while dragging its feet on allowing the U.S. to deploy an advanced missile defense system on its soil despite objections from Beijing.

Seoul officials have downplayed the meaning of Russel’s remarks, saying they didn’t think he was calling for a new measure by the South Korean government. They appear to want to believe he was expressing a general view of the country’s role in global affairs in terms of universal principles and international norms.

But it is unpersuasive to interpret what he meant just by his wording. His request may have to be seen as a thinly veiled demand that South Korea depart from its strategic ambiguity and stand by the U.S. in keeping a rising China in check. In his remarks to last week’s seminar, Russel said Beijing’s behavior in the South China Sea raises the question: What kind of a power does China seek to become?

In the eyes of diplomatic observers here, it did not seem coincidental that Russel made the request ahead of President Park Geun-hye’s planned visit to the U.S. next week. Her summit with U.S. President Barack Obama is expected to focus on deterring nuclear and missile threats from North Korea. But there is still the possibility of the situation in the South China Sea being discussed at their meeting as a key regional security issue.

In dealing with the sensitive matter, South Korea needs to take an approach more substantive than strategic ambiguity, which could help consolidate its alliance with the U.S. while avoiding denouncing China in an explicit manner. It may serve this purpose to raise the need ― in a more equivocal way ― for China to be faithful in implementing a 2002 deal with Southeast Asian nations on their conduct in the South China Sea.