GENEVA (AP) _ A mountain lake held back by an unstable natural dam in Central Asia could cause a huge natural disaster, experts say. No one is certain how to prevent it.

Lake Sarez lies high in the Pamir Mountains of Tajikistan, an area that has been rattled by at least three dozen earthquakes since 1990. The 37-mile-long lake itself was formed by a quake in 1911.

If the dam were to break, experts say a wall of water would threaten parts of Tajikistan, Afghanistan, Uzbekistan and Turkmenistan.

The U.N. Department of Humanitarian Affairs, a Geneva-based group that coordinates U.N. relief operations, says such an event ``could easily become the deadliest natural disaster in history.''

Although predicting the precise effect of a dam break is difficult, international experts are urging governments to take action to prepare.

``The problem is major and time presses forward,'' said Yuriy Kazakov, a geological expert with KAM, a New York-based environmental group.

The freshwater lake, located 225 miles east of the Tajik capital, Dushanbe, was the major topic at three international disaster-reduction conferences last year.

In an earthquake, the experts say, an unstable cliff above the lake could collapse and throw a huge wave of water over the top of the dam. Alternatively, the dam itself could break up.

``If a powerful earthquake occurs _ and it will definitely occur because quakes happen there all the time _ rocks will collapse and this mass will fall into the lake, pushing the water out,'' said Samuel Grigorian, a Moscow State University professor.

At least 100 earthquakes have hit the region in the past century, say officials in Tajikistan, a former Soviet republic.

In April 1997, a magnitude-7 quake centered 60 miles southwest of Dushanbe destroyed more than 1,000 buildings and killed two people.

Quakes as strong as magnitude-9 have been recorded in the area, said Dr. Arkady Sheko, a member of the Russian Academy of Science and head of geology at the All-Russian Institute for Geology and Engineering Technologies.

If the dam were destroyed, the whole lake could spill out and threaten a 20,000-square-mile area inhabited by 5 million people, Grigorian told The Associated Press. Some 1,500 people live directly below the lake in the Murgab gorge, he said.

With the nearest villages 19 miles from Sarez, a flood wave moving about 16 feet per second would reach them in less than an hour, Sheko said.

``They would all be killed. Everything would be swept away _ everything standing in the way of a flood,'' Grigorian said.

He cited an accident at another lake, Lake Issyk in the Central Asian republic of Kazakhstan, in the 1970s, where similar conditions caused a flood that broke down all dams and totally emptied the lake. Several people were killed, although Issyk was much smaller than Lake Sarez, he said.

Sheko said it would be impossible to strengthen the dam enough to resist the kind of wave that would be caused if the cliff or other massive rocks collapsed into the lake.

``There are rocks the size of five-story buildings, whole massifs,'' said Sheko, who says he was the first to alert the Soviet government to the danger in the 1960s.

Lowering the lake's water level could solve the problem, but that would cost millions of dollars, Sheko said.

All equipment would have to be flown in because there is no land access and a pipeline would be needed to remove the excess water.

``This is an enormous engineering problem,'' said Behrooz Ross-Sheriff, chief executive officer of Focus on Humanitarian Assistance, a U.S. relief organization that has been involved in the research.

Lake Sarez, at 10,700 feet above sea level, was formed in 1911 when an earthquake caused a landslide that blocked the Murgab River, one of the tributaries of the Amu Darya River.

Agencies including the International Organization for Migration, an aid group that cares for refugees, hope to raise $140,000 to carry out further studies and plan an evacuation strategy.

``There are clear danger signs, but the international community is not ready to take up this problem at the moment,'' agency spokeswoman Claire Messina said.

Editor's note: Anna Dolgov of the AP's Moscow bureau contributed to this report.