The US has expressed concern over China’s recent behavior towards Philippine vessels in the South China Sea, with Defense Secretary Lloyd Austin calling Beijing’s move “coercive and risky.”

Austin, in a readout released Thursday after a call with Philippine counterpart Gilberto Teodoro, also reiterated the US’ “ironclad” commitment to defend the Southeast Asian nation. The two countries’ mutual defense treaty covers Coast Guard vessels in the South China Sea, Austin said.

The Philippine Coast Guard said earlier this week that its vessels were “constantly followed, harassed and obstructed” late last month by bigger Chinese ships in the South China Sea. China’s Foreign Ministry responded by saying its ships’ maneuvers were “professional and restrained.”

Chinese and Philippine ships nearly collided in the disputed sea in April, with both sides accusing one another of intrusion and provocation. Philippine President Ferdinand Marcos Jr. said last month that he sees Beijing as a partner in promoting peace and trade in the South China Sea, amid his government’s recent push to bolster defense ties with the US.

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The Chinese Coast Guard’s recent moves were “unprofessional,” and Beijing’s “irresponsible behavior” in the South China Sea threatens the Philippines’ security, Washington’s envoy to Manila MaryKay Carlson said in a separate Twitter post on Friday.

The Philippines has filed 97 diplomatic protests against China since Marcos took power a year ago, according to Philippine Star.