A television crew from Philippine broadcaster ABS-CBN was travelling to Second Thomas Shoal in the contested Spratly Islands on Thursday when their vessel was allegedly chased by a Chinese Coast Guard boat and two attack craft.

Tensions over the resource-rich sea have spiked in recent weeks after hundreds of Chinese vessels were detected at Whitsun Reef, which is also in the Spratly archipelago.

China, which claims almost the entirety of the sea, has refused repeated appeals by the Philippines to withdraw the vessels, which Manila says unlawfully entered its exclusive economic zone (EEZ).

"The AFP expresses concern over the report of an alleged harassment by Chinese PLA Navy vessels and a Chinese Coast Guard vessel as reported by a team of journalist(s) on board a watercraft within our EEZ," said Major General Edgard Arevalo, spokesman for the Armed Forces of the Philippines.

"Through the AFP's Western Command, we are conducting investigation and verifications to establish what transpired."

ABS-CBN said the vessel carrying its TV crew was asked by a Chinese Coast Guard boat to identify itself. The Filipino boat turned around and the Chinese vessel gave chase, it said.

"CCG 5101 slowed down and turned away after an hour to the relief of the Filipino crew, who by this time had been following a straight path back to mainland Palawan," ABS-CBN reported, referring to the western Philippine island.

"However, two smaller, faster vessels emerged in the horizon, apparently giving chase to the Filipino boat," it said.

"Within minutes, the unique shape and design of the Houbei Type 22 missile fast attack craft became visible. The two missile-capable boats resumed the chase."

Foreign Secretary Teodoro Locsin -- whose department vowed recently to lodge a daily diplomatic protest until China withdraws its vessels from Philippine waters -- said the report was "worrisome".

"It seems - please verify - ABSCBN crew weren't in a boat they hired (inviting risk) but in a passenger boat," Locsin said on Twitter.

"Now that's a whole other banana; the state is sworn to protect the Philippine public. And you don't give chase; you use your bullhorn to warn off."

In a phone call on Friday, Locsin and his US counterpart Antony Blinken expressed concern over the presence of Chinese "maritime militia vessels in the South China Sea, including at Whitsun Reef", according to the US State Department readout.

The United States reminded China on Wednesday of Washington's treaty obligations to the Philippines in the event of an attack in the waters.

Chinese foreign ministry spokesman Zhao Lijian said Friday the United States should "stop sowing discord" and allow regional countries to settle "their differences between themselves".