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.