GEOFF BENNETT: And we start the day's other headlines in a border area of Nepal and Tibet where flash floods have left a trail of death and destruction.

Authorities say at least 160 people have died, hundreds more are still missing.

A warning: Some footage in this report may be disturbing.

It came without warning.

The silence of this security footage belies the panic at a border crossing between Nepal and China's Tibet region, dozens of people running for their lives before a towering wall of mud and water barrels through, swallowing nearly everything in its path.

The torrent tossed buses aside like toys and swept away buildings and parking structures.

MAN: Chinese President Xi Jinping has ordered all-out efforts to search for and rescue people who are missing.

GEOFF BENNETT: The full toll is unknown.

Chinese state media reported significant casualties with hundreds of people still unaccounted for.

The flash floods tore through the steep Himalayan valleys along the border between China and Nepal, with floodwaters surging south toward the Kathmandu region.

Satellite images reveal the scale of the destruction, entire stretches of the landscape transformed.

Scientists studying the disaster say it may have started when a massive chunk of ice snapped off a glacier high in the Himalayas, unleashing a devastating cascade into the valley below.

Eyewitness footage captured the torrent slamming into Nepal's Nuwakot district with terrifying force.

Some residents say they escaped with only minutes to spare.

SHOYAM RAJBHANDARI, Nuwakot, Nepal, Resident (through translator): When I reached the bazaar, in half-an-hour, the flood came and damaged all of the market.

People were preparing to go to the office then.

How many people are missing or dead?

We don't know.

GEOFF BENNETT: From above, the scale of destruction came into sharper focus, buildings buried beneath the vast new landscape of mud and debris.

As crews searched the wreckage, families waited desperately for word about missing loved ones.

Nepalese authorities say hundreds of foreign nationals are among the missing, including dozens of Americans, but amid the destruction, remarkable rescues, including this man pulled to safety, others caked in mud rushed by rescuers to nearby hospitals.

And some survivors linked arms as they made their way to safety, trudging through waist-deep mud.

And, tonight, the U.S.

State Department says it's aware of Americans affected by the flooding and is working with local authorities to assist them, adding that the U.S.

stands ready to provide assistance to Nepal.