Another image, taken in daytime, showed heavy smoke billowing to the sky and fire still burning.
The fire broke out in "one of the systems" of the ship, a navy statement said without elaborating.
The expedition forces tried to extinguish the fire of Khark ship for 20 hours without interruption, but the fire was so intense that half of the ship's hull melted and sunk later off, said Behzad Jahanian.
The ship's crew all safely disembarked.
Iran's army also identified the Kharg as a "training ship" and confirmed there were nearly 400 crew and trainees on board, before adding all of them were safely evacuated. [Asriran.com via AP] The vessel was built in Britain and launched shortly before Iran's 1979 Revolution and entered the Iranian navy in 1984 after several years of negotiations. It's also one of the only vessels in the Iranian navy capable of providing replenishment at sea for its other ships.
"The Iranians often describe it as a "helicopter carrier" but it is actually a replenishment ship - a useful asset nonetheless as it was the Islamic Republic of Iran Navy's only dedicated ship serving this role", Binnie said.
"It is not the IRIN's largest vessel any more after they commissioned Makran, a converted oil tanker, in January".
The navy said the vessel was on a "training mission" in worldwide waters.
State TV and agencies referred to the Kharg as a "training ship".
The vessel sank close to the port of Jask in the south of the country, near the Strait of Hormuz, a strategic waterway.
An approximately 1,000-kilometre (620-mile) pipeline from Bushehr province on the Gulf to Jask was put into service a few days ago, the government said.
It is all a far cry from the confident attitude Iran was showing previously, as Breitbart News reported.
In April, an Iranian ship called the MV Saviz believed to be a Guard base and anchored for years in the Red Sea off Yemen was targeted in an attack suspected to have been carried out by Israel.
That came after Israel and Iran had blamed each other for a series of reported attacks on cargo ships since late February.