WASHINGTON (REUTERS) - The national security advisers for the United States and the Philippines discussed their shared concerns over Chinese activities in the South China Sea in a call on Wednesday (March 31), the White House said.

The Philippines has described the presence of hundreds of Chinese boats inside its 200-mile (321km) exclusive economic zone at Whitsun Reef in the South China Sea as "swarming and threatening." Manila believes the vessels were manned by maritime militia.

Chinese diplomats have said the boats were sheltering from rough seas and no militia were aboard.

White House national security adviser Jake Sullivan and Philippines national security adviser Hermogenes Esperon "agreed that the United States and the Philippines will continue to coordinate closely in responding to challenges in the South China Sea," the White House said.

"Sullivan underscored that the United States stands with our Philippine allies in upholding the rules-based international maritime order, and reaffirmed the applicability of the US-Philippines Mutual Defence Treaty in the South China Sea," it added.

Canada, Australia, Japan and others have voiced concern about China's intentions.

Brunei, Malaysia, the Philippines, Taiwan, China and Vietnam have competing territorial claims in the South China Sea, through which at least US$3.4 trillion (S$4.5 trillion) of annual trade passes.