Ukraine's new missile sank the biggest warship in combat since WWII

• The Russian warship, Moskva, was hit by at least one Neptune cruise missiles before it sank, according to a senior U.S. defense official.
• The Neptune is Ukraine's first domestically-made, medium-range cruise missile designed to target warships.

The Russian Black Fleet flagship, Moskva, that sank on Thursday, was hit by a new type of Ukrainian missile, according to a senior U.S. defense official who briefed reporters on Friday. Russia has maintained the Moskva was damaged in a fire.

The senior U.S. defense official said the ship was hit by at least one Neptune missile and that there are likely casualties — Russian authorities said the crew, believed to be around 500 sailors, was evacuated when a fire broke out and triggered ammunition explosions. The extent of the injuries and deaths is not publicly known.

This would be the first known use of the Neptune missile system and Ukraine's most significant naval strike in the war against Russia.

The Neptune is the first domestically-made medium-range cruise missile for Ukraine, designed to be used against warships, like the Moskva. It is an update of the Soviet-era KH-25 missile — which could only be fired from ships and planes — modified to be fired from truck-mounted launchers at targets on land and sea. It has a range of around 200 miles and is mainly designed to target warships like cruisers and destroyers.

The final tests of the Neptune were carried out in late February, as the war broke out but Ukrainian officials were doubtful of the missile system's readiness, according to a report from the Wall Street Journal. They had originally estimated the Neptunes would be deployed to coastal cities, like Odessa, by April 2022.

Russia has superior naval capability in the Black Sea, and officials hoped the Neptune might level the playing field by raising the cost Russia would pay for shelling Odessa and other vulnerable coastal cities.

Andriy Zagorodnyuk, former Ukrainian defense minister, told the Wall Street Journal that the Neptune allows Ukraine to "hit any target within our territorial waters."

"Essentially, this is about being able to block the capabilities of the Russians if they try to attack," Zagorodnyuk told the Wall Street Journal in February.

The Moskva is the largest warship to be lost in combat since World War II. The closest ship by size to the Moskva is the Argentine light cruiser General Belgrano that was torpedoed by a Royal Navy submarine in 1982 amid the Falklands war.

On Thursday, the Russian Defense Ministry had claimed the flagship was still floating and that the cause of the fire was "yet to be established," according to the state-run news agency TASS. Questions remain about the sequence of events that sank the ship, with speculation that the ammunition explosions may have breached the ship's hull and led to flooding the crew couldn't contain.

Ukrainian presidential adviser Oleksiy Arestovych praised the strike on Thursday on Telegram, calling it a "hugely important military event and the biggest defeat of the Russian Navy since World War II."