Established as an Ottoman palace, then a residence for one of Yemen's last kings, the building became a museum in 1967 but has since been bombed and its collections pilfered.
It was partially restored in 2019, but a recent deluge caused the entrance hall to give way last week, with the facade and the rooms inside reduced to rubble.
"One of the main entrances collapsed... as a result of the heavy rainfall which poses a threat to all cultural and historical sites," the museum's director Ramzi al-Damini told AFP, urging the international community to intervene to protect Yemen's heritage.
The bloody five-year-old conflict pits pro-government forces, backed by a Saudi-led military coalition, against the Iran-backed Houthi rebels who have conquered much of the country's north, including the capital Sanaa.
Taez, a city ringed by mountains, is under government control but surrounded by rebel forces whose bombardments have hit the museum several times.
Read also: Yemen's terrifying road to Taez on brink of collapse
Yemen's rich cultural heritage has been among the casualties of the conflict that has left thousands dead, millions displaced, and cities and villages stalked by famine and disease in what the UN calls the world's worst humanitarian crisis.
"I feel despair and sadness to see my country's heritage destroyed, our civilisation destroyed, our history destroyed," said Akram al-Hmeiri, a resident of Taez.
"Everything in Yemen has been destroyed," he told AFP.
The renovation work on the National Museum had left parts of the complex beautifully restored, with the facade, intricate ochre brickwork, and arched windows back to their original grandeur.
Other parts however remained pocked with damage, crumbling away to reveal collapsed floors and shattered walls.
The disrepair left it vulnerable to the heavy rains which hit Yemen in recent months, triggering flash floods that killed scores of people and damaged other UNESCO-listed World Heritage sites in the capital and in the cities of Zabid and Shibam.
Taez's museum had planned to re-open its doors in 2023, by which time officials hope the conflict will have abated.
"If the damage to the museum is not fixed and renovations undertaken... it will have a negative effect on the city," said Ahmed Jasar, the region's director of antiquities.
"The museum is symbol for Taez," he told AFP.
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