Manila and Beijing traded accusations about a maritime confrontation Wednesday around the disputed Scarborough Shoal, a day after China submitted a chart of the South China Sea feature’s shore to the U.N. to bolster its claim over it.

China has blocked four Philippine vessels that attempted to “intrude into the territorial waters” of the disputed Scarborough Shoal on Wednesday, the country’s coastguard said.

Chinese incursions at Scarborough Shoal and Pag-Asa Shoal (Thitu Island) in the Philippines expose legal warfare over Philippine claims in the South China Sea.