This image from video provided by US Defense Department shows explosion on an Iranian ship in the Indian Ocean during the joint US-Israeli military operation against Iran. Photo: VCG

The US claimed Wednesday that a US submarine sank an Iranian warship in international waters, a move a Chinese military expert said was a dangerous sign that the joint US-Israeli military operation against Iran is expanding to a larger scale.

According to a CNN report, US Secretary of Defense Pete Hegseth made the claim, with the Pentagon releasing a video of the attack showing a ship that experiences a massive explosion by its stern as well as what appeared to be still frames showing the ship sinking.

The attack killed more than 80 crew according to authorities in Sri Lanka who launched a rescue mission, and the Iranian side identified the vessel as the IRIS Dena and vowed vengeance for what it described as an "atrocity," CNN reported.

Some 130 people were believed to have been on board the vessel when the first distress call was received on Wednesday, according to Iran's foreign minister Abbas Araghchi, according to CNN.

"The US has perpetrated an atrocity at sea, 2,000 miles away from Iran's shores," Araghchi said on X. "Mark my words: The US will come to bitterly regret precedent it has set."

The IRIS Dena had been sailing home from an east Indian port, where it had participated in an international naval conference hosted by India in February.

US' Chairman of the Joint Chiefs of Staff Gen. Dan Caine claimed that it was the first time a US attack submarine had used a torpedo to sink a combat ship since 1945, CNN reported.

Wang Yunfei, a Chinese military affairs expert, told the Global Times on Thursday that the US nuclear submarine must have arrived in international waters near India and Sri Lanka where the attack was launched. This indicates that the US had planned the operation in advance. The ambush cannot be a sudden decision.

Wang warned that the US attack in international waters is an indication that the ongoing joint US-Israeli military operation against Iran could be expanding. "In response, Iran could strike more US assets around the world."