Nyan Win, a veteran senior member and former spokesperson of the National League for Democracy (NLD) party -- led by Suu Kyi -- was 78.

He had been arrested after the February 1 coup removed the NLD from power and was held in Yangon's notorious Insein prison on charges of sedition.

"U Nyan Win was found with Covid symptoms on July 11 and transferred to Yangon General Hospital... to get treatment," said Zaw Min Tun, spokesman of the State Administration Council -- as the junta calls itself.

"He died this morning at 9 am in the hospital."

Nyan Win had underlying conditions of hypertension and diabetes, he added.

A veteran politician who worked on human rights issues, Nyan Win was the sole person allowed to meet with Suu Kyi during the previous junta regime when she was held under intermittent house arrest for 15 years.

He served as a conduit for the Nobel laureate while she was imprisoned, passing on her messages from an isolated Burma -- as Myanmar was formerly known as -- to the outside world and her supporters.

"We have relied on him so much. I am so sad to have lost him," lawyer Khin Maung Zaw, who is part of Suu Kyi's legal team, told AFP.

"But we will transform our sorrow into our strength to move forward."

Myanmar -- which has been in turmoil since the coup -- is now buckling under a surge of Covid-19 cases.

Hospitals across the country are suffering from a lack of medical equipment, beds, oxygen and doctors.

Myanmar's prisons are also full, due to the junta's crackdown on dissidents, activists and NLD politicians -- raising concerns of a fatal outbreak behind bars.

Spokesman Zaw Min Tun said 375 people in prisons across the country have tested positive for Covid-19. About 200 of them are in hospitals, including senior NLD member Han Thar Myint who is currently in intensive care.

American journalist Danny Fenster is also being held at Insein after he was detained while trying to leave Myanmar in May.

State-run media said that 1.6 million people have been vaccinated across the country -- about two percent of the population.