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

• China has ramped up actions towards the Philippines in a disputed shoal in the South China Sea, using "gray zone tactics" to skirt a U.S.-Philippines defense pact.
• Other nations will watch the U.S. response as an indicator of American deterrence strength in the South China Sea, experts say.

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