China is testing the limits of a critical U.S.-Philippines defense pact — will Washington respond?

China has stepped up its aggression against the Philippines in the contested waters of the South China Sea, calling into question the strength of American deterrence, according to policy analysts.

Last week, the Chinese coast guard seized two Filipino ships on a resupply mission to an outpost on the Second Thomas Shoal in the Spratly Islands, severely injuring a navy personnel, according to Philippine officials. The shoal is claimed by both Manila and Beijing.

The area has seen several clashes over the past months. Experts say the latest incident represents an escalation and shows the limitations of the U.S.-Philippines Mutual Defense Treaty signed in 1951.

"The failure of the MDT to deter the [latest confrontation] shows the vagueness in the conveyed commitments of the two parties," said Chester Cabalza, president and founder of the Philippines-based think tank International Development and Security Cooperation.

Last year, the Philippines and U.S. released new "Bilateral Defense Guidelines ," reaffirming that an "armed attack" in the South China Sea on Filipino vessels would invoke mutual defense obligations of the U.S.

China, on its part, has been careful not to trigger the MDT by avoiding the use of guns, instead employing " gray zone" tactics — coercive actions that fall short of an armed conflict — at the Second Thomas Shoal. These reportedly have included the use of water cannons and ramming into Philippine boats.

Beijing's actions aim to stop resupply missions to a rusted Philippine vessel grounded on the shoal since 1999, as its existence there appears to lend credence to the Philippines' claim on the reef.

According to Cabalza, unless Manila and Washington do more to increase deterrence, Beijing will continue to employ "gray zone" tactics in its attempt to win the shoal and other areas contested by the Philippines.

"The usage of these tactics should be classified as an armed attack [in the MDT] if the U.S. is serious about helping the Philippines in its strategic and asymmetrical warfare with China," he said.

An expanded definition of an "armed attack" in the treaty could include "any willful act to inflict injury or that leads to fatality on Filipinos," said Richard Heydarian, policy adviser and senior lecturer of international affairs at the University of the Philippines.

Those guidelines are consistent with the interpretation of the MDT in a report by the U.S. Indo-Pacific Command that was unclassified last year.

However, according to Heydarian, a muted Philippine reaction to last week's clash could be a sign of its apprehensions with regard to invoking the mutual defense treaty.

"The Philippine Government is confronted with a huge dilemma. It wants greater reassurance from the United States, but it's likely not getting that, and it also wants to avoid unwanted escalation," said Heydarian.

"It's extremely important for the Philippines and U.S. to signal that Filipino troops are empowered and sanctioned to use live firearms to defend themselves, and that usage of live firearms by China would immediately trigger the MDT," he added.

However, according to Abdul Rahman Yaacob, a research fellow at the Lowy Institute's Southeast Asia Program, the U.S. is unlikely to widen the scope of the defense pact, as doing so could draw them into a wider military conflict.

Amid a "sober assessment on the risk of U.S. non-involvement," the Philippines will also look to step up its own deterrence and strategy in asserting its territorial claims, according to Matteo Piasentini, China and Indo-Pacific analyst at Geopolitica, an Italian think tank.

Philippine President Ferdinand Marcos Jr. said on Thursday that while the latest incident at the shoal was not an "armed attack," the country needs to "do more" than protest China's actions.

"The Philippines will continue to resupply the Sierra Madre, and hopefully invest in more outposts in their Exclusive Economic Zone in the South China Sea ... There is no illusion that China will give up; what Manila is trying to do is not to give up on its own," Paisentini said.

The Philippines defense chief has reiterated that the military would not announce its resupply missions to the shoal in advance. According to the Lowy Institute's Yaacob, this could help it outmaneuver Beijing while avoiding direct confrontations.

The country has also been working to bolster its own defense capabilities in the South China Sea with the support of the U.S.

The Second Thomas Shoal is emblematic of how great powers are becoming involved in the security architecture in Asia, with states such as the Philippines increasingly turning to the U.S. and other powers to increase deterrence and security, according to Geopolitica's Paisentini.

"Non intervention [ if China gets more aggressive] would cause a massive blow not only in U.S.-Philippines relations, but also in the relationship between the U.S. and other key regional allies," he added.  

From China's viewpoint, the shoal represents its ability to enforce territorial claims under its "9-dash line" and to delegitimize rulings from organizations such as the UNCLOS, which determined the Second Thomas Shoal to fall under Philippine sovereignty in 2016, experts told CNBC. 

It also wants to prevent the U.S. from using the Philippines as a critical part of the "first island chain"- a chain of islands encompassing portions of Japan, Taiwan, the Philippines and Indonesia- seen as strategically important to contain the reach of the PLA, said researcher Muhammad Faizal.

The research fellow at Singpore-based S. Rajaratnam School of International Studies added that "every ally and partner is probably watching how committed the U.S. really is to its military alliances and planning ahead if the commitment falters."