President Joe Biden will warn China about its increasingly aggressive activity in the South China Sea this week during summits with Japanese Prime Minister Fumio Kishida and Philippines President Ferdinand Marcos Jr. 

Two senior US officials said Biden would express serious concern about the situation around the Second Thomas Shoal, a submerged reef in the Spratly Islands where the Chinese coast guard has used water cannons to prevent the Philippines from resupplying marines on the Sierra Madre, a rusting ship that has been lodged on the reef for 25 years. 

Biden will stress that the US-Philippines Mutual Defense Treaty applies to the Sierra Madre, said the officials, adding that he expressed “deep concern” when he spoke to President Xi Jinping on Monday.