WASHINGTON (AP) — In 1988, Omar Khan Masoudi huddled with other museum curators in Afghanistan to make a critical decision — how to protect some of the nation's oldest and most prized artifacts.

Masoudi and others from the National Museum of Afghanistan began to worry as security in the capital of Kabul crumbled and the Soviet Union made plans to withdraw its troops.

They arranged to move several collections — with artifacts dating back some 4,000 years — into underground vaults at the presidential palace and to another secure site.

They took a vow of silence that proved heroic when the museum was slowly demolished, starting with a rocket strike in 1994 and ending with the Taliban regime's destruction of thousands of artworks in 2001.

That year, the Taliban also dynamited the giant 1,500-year-old Buddha statues carved in the Afghan cliffs at Bamiyan.

"The museum staff, they were really honest people," said Masoudi, the museum's director, recalling their fears under the Taliban. "We never gave anybody any information at that time, because there would be danger."

The Taliban came across the vaults. But curators told them and others that the keys had been lost and that they didn't know what was inside.

In 2003, after the Taliban was removed from power, Afghan President Hamid Karzai announced the vaults had been "discovered".

The next year, artifacts made of gold, ivory, glass and other valuables from the Silk Road trade routes of Asia, were finally retrieved.

These treasures, including intricate gold crowns and ancient sundials, are now featured in their first exhibition in the United States.

The exhibit "Afghanistan: Hidden Treasures From the National Museum, Kabul" opens Sunday at the National Gallery of Art in Washington.

"Finally, we can tell this story," said Fredrik Hiebert, a National Geographic archaeologist who conducted an inventory of the ancient objects and organised the exhibit. "After all these years, these boxes were saved by secrecy."

The story about the lost keys was indeed true. But a curator also intentionally broke one key to jam a vault's lock.

The only way to open the vaults was to use a large circular saw — though some feared the heat and sparks from the saw would melt some of the precious gold inside.

"But there, when the safe was opened, there it was," Hiebert said, known as National Geographic's own Indiana Jones in some circles.

Other, unexpected treasures were found in the vaults as well, he said, including a plaster medallion thought to be stolen or destroyed and 2,000-year-old, coloured-glass fish.

These collections, which come from four archaeological sites in Afghanistan, will be on display through the summer in Washington before travelling to the Asian Art Museum of San Francisco, the Museum of Fine Arts, Houston, and the Metropolitan Museum of Art in New York.

The exhibit is organised by site. It begins with the Bronze Age city of Tepe Fullol in northern Afghanistan, featuring 2200-1900 B.C. artifacts such as gold bowls that feature animals, trees and mountains.

Bronze and ivory Greek sculptures and Corinthian columns are among the artifacts from the former Greek city of Ai Khanum — a region conquered by Alexander the Great.

Those objects, which date back to the third century B.C., also include sundials and a carved portrait from the city's gymnasium in the Greek tradition of athletics and academics.

The third section revolves around the ancient city of Begram, where elaborate carvings of Indian ivory and decorative figurines emerged.

Perhaps the most impressive artifacts, though, are found in the final section, which showcases about 100 gold ornaments and pieces of jewellery discovered in 1978 by a Soviet-Afghan team at Tillya Tepe in ancient Bactria, located in present-day northern Afghanistan.

Archaeologists uncovered the graves of six nomads, who were adorned with thousands of gold objects sewn onto their burial gowns and suits.

Jewellery with turquoise and garnets were also found at the burial site.

One of the nomads — a woman in her 20s — wore an elaborate gold collapsible crown with tree and bird designs.

Of the five women buried there, she had the highest rank. Other items in her grave included a gold sceptre and a Chinese bronze mirror.