Amid the raging war and constant threat of Russian missiles, a successful heart transplant has been performed on a 6-year-old girl in Kyiv, authorities with the Heart Institute of Ukraine's Ministry of Health announced on Monday.

The three-hour operation, which took place on Sunday evening, gave the girl the heart of a 4-year-old boy, whom doctors had declared brain dead after suffering an aneurysm.

It was the first time a heart transplant had been performed in Ukraine on children so young, the institute said.

"The operation was also unique in that both the donor and the recipient were very young children, and the transplant required more effort from the doctors," it added.

The transplant was performed by a team of doctors led by Dr. Boris Todurov, the chief scientist of the department of surgical and minimally invasive treatment.

"The operation went smoothly, the girl was extubated two hours after the operation," Todurov said in a post on his official Facebook page.

The Heart Institute released images from the operation showing the mother of the boy whose heart was donated standing by the girl's bedside.

"The operation went well, and the new heart is beating in the girl's chest," Oksana Dmytrieva, chairman of the Ukrainian Parliament's subcommittee on modern medical technologies and transplantation development, said in an emotional post on Facebook.

"It is extremely touching that the mother of the deceased boy came to listen to her child's heart beating in the other chest ... I have tears in my eyes from this photo," she added.

Three more of the boy's organs -- two kidneys and a liver -- were transplanted to two other children at another hospital in Kyiv, the National Children's Hospital "Ohmatdyt."

The two kidneys were transplanted to a 12-year-old boy from the occupied part of Kherson region. "He had been waiting for a transplant for more than 3 years and lived at the Ohmatdyt," the hospital said in a Facebook post. A liver was also transplanted to a 15-year-old boy from Kirovohrad region.

"Many thanks to the parents of the little angel, who made an extremely noble act in this extremely difficult time for the family! You are people with a big and kind heart!" Dmytrieva said. "May the little angel rest in peace. And a speedy recovery to the patients who got a chance for a full life!"

Transplants would not be possible without the relatives of donors making the decision "to save the lives of people they do not know after losing a loved one," Dmytrieva said. "This is the noblest manifestation of humanity. Especially when it comes to the loss of a child."

Heart surgery during wartime

So far this year, 23 heart transplants have been performed in Ukraine amid the war, the Heart Institute said.

If an operation is already underway and air raid systems are activated, the operation cannot be interrupted and will continue even if there is an attack on the city, the institute told CNN on Monday.

If the operation hasn't started, the team of doctors and the patient wait for the air raid sirens to stop before beginning the operation.

Earlier this year, when Kyiv was less protected by air defenses, all patients were brought down to a specially equipped bomb shelter.

Those who could move around went on their own, while those who could not were helped by staff, the Heart Institute said.

The only exception was patients in the intensive care unit, who could not be moved, so the doctor on duty and other medical staff would stay with them.

The Heart Institute has purchased special generators so operations can continue during blackouts, and it has an autonomous water supply.

During the war, the institute said it had seen more patients suffering from cardiovascular diseases and worsening chronic diseases due to stress.