The US military has released dramatic footage of the moment one of its submarines torpedoed an Iranian warship.
The Iranian naval vessel was situated off the coast of Sri Lanka, in international waters and far from the regime's territory, when the US sub opened fire.
Footage released by the Pentagon shows the warship on the water one moment, before a huge explosion rips through one half of the vessel.
It marks a dramatic escalation of the US military's pursuit of Iran's navy.
Here's what we can tell you about this incident so far.
Which Iranian ship was sunk?
The vessel hit was the IRIS Dena, which US Defense Secretary Pete Hegseth says was the Islamic Republic's "prize ship".
It was one of Iran's newest warships, patrolled in deep water, and was armed with heavy guns, surface-to-air missiles, anti-ship missiles and torpedoes. It carried one helicopter.
The vessel was situated off the coast of Sri Lanka, and local authorities there say they have recovered 87 bodies and rescued 32 people.
It is believed 180 sailors were on board the Dena when Sri Lanka's navy first received a distress signal on Wednesday, local time, and 60 remain unaccounted-for.
By the time Sri Lanka's navy reached the location, there was no sign of the ship, "only some oil patches and life rafts," navy spokesman Commander Buddhika Sampath said.
"We found people floating on the water."
[EL DW: Submarine sunk]Why did the US sink the Iranian ship?
Both the US and Israel are seeking to hobble Tehran's ability to fight by any means necessary, and have been systematically taking out Iran's naval fleet in recent days.
US President Donald Trump has publicly declared that wiping out Iran's navy is one of his chief war aims.
That could be because of Iran's location next to one of the world's most valuable shipping routes.
Tehran has sought to use its naval, ballistic and drone fleets to close the Strait of Hormuz, in a move that threatens to throttle the global shipping of oil.
At the time of its sinking, the Dena had been heading back to Iran from eastern India.
What have the US and Iran said?
Mr Hegseth has declared the Dena's sinking a major blow to Iran's navy.
"An American submarine sunk an Iranian warship that thought it was safe in international waters," he told a Pentagon news briefing.
"Instead, it was sunk by a torpedo."
What remains of Iran's leadership, six days into a war against the US and Israel, is yet to publicly comment on the sinking.
What will happen to the injured sailors?
Sri Lanka's Foreign Minister Vijitha Herath told the country's parliament ships and planes had been dispatched to rescue the stranded sailors.
Pictures showed ambulances ferrying those wounded to a local hospital in the town of Galle, near where the ship was sunk, while the bodies of those killed were taken to a nearby mortuary.
Donald Rothwell, who is a professor of International Law at the ANU College of Law, told ABC News the US did have a right to target the vessel under rules of engagement.
But he added that the US's decision to torpedo the vessel, far from Iran's territorial waters, marked an expansion of the conflict's battle zone.
"This is a significant escalation by the United States in terms of the area of combat operations extending well into the Indian Ocean and quite some considerable distance from where we've seen the conflict being seen centred at the moment in the Middle East," he said.
How many warships does Iran have?
A report by the Australian Naval Institute last year found that Iran's navy consisted of "over 18,500 personnel and more than 100 vessels, including ships and submarines".
While significant, that is nowhere near the firepower that can be mustered by the US Navy, which alone has 11 aircraft carriers, two of which are deployed to this Middle East war.
And the US and Israel are now relentlessly targeting Iran's vessels.
US General Dan Caine said the Pentagon had destroyed 20 of the regime's vessels since the US and Israel began carrying out strikes on Saturday.
Satellite images taken of Iranian naval bases show vessels either sunken or listing after being struck by US and Israeli forces.
When did a US submarine last sink a ship?
Defense Secretary Hegseth said it was the first time a submarine had sunk a warship since World War II.
"Like in that war," Hegseth said, "we are fighting to win."
But that claim is being contested, with at least one other example being the British HMS Conqueror having sunk an Argentine ship during the Falklands War in 1982. A Pakistani submarine also torpedoed an Indian frigate in 1971.
ABC/wires
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