Iraq's Arch of Ctesiphon, the world's largest brick-built arch, is having restoration work to return it to its former splendour, authorities say.
The sixth-century monument, located around 30 kilometres south of the capital Baghdad, is the last structure still standing from the ancient Persian imperial capital Ctesiphon.
Restoration work on the arch, also known as Taq Kasra, was carried out in 2013 after heavy rain caused a massive slab to fall.
However, the new bricks also began to fall following downpours last year.
The first phase of “emergency” work that began in March is due to end next month, said David Michelmore, a conservation expert working with a team of archaeologists from the University of Pennsylvania.
“What is falling down at the minute is not the original Sassanian construction, it's the modern repairs,” he told AFP.
“There was quite a lot of reconstruction done in 2013-2014 and probably all of this will need to be taken down and replaced."
Construction of the arch began in AD540 during the Persian Sassanid dynasty's long wars with the Byzantine Empire. It formed part of a palace complex started three centuries earlier.
It stands at 37 metres tall and 48 metres long.
Iraqi Culture Minister Hassan Nazim said the work aimed to “consolidate” the site, which is near the bank of the Tigris River and is at risk of groundwater infiltration.
The current phase is financed thanks to a budget of $700,000 from the International Alliance for the Protection of Heritage in Conflict Areas, said Laith Majid Hussein, director of the Iraqi State Board of Antiquities and Heritage.
He said “numerous mistakes” were made in the previous restoration, including the installation of a heavy “layer of cement on the arch".
The next stage would be “total restoration” that would help strengthen the structure, he said.