Rescuers saved more people from underneath the rubble of collapsed buildings Thursday. Still, hopes are fading of locating many more people alive more than three days after catastrophic earthquakes and a string of aftershocks hit Turkey and Syria, killing more than 18,000.

The U.S. Embassy in Ankara, Turkey, confirmed Thursday that at least three Americans were among the dead in that nation.

Experts said the survival window for those trapped under the rubble or otherwise unable to obtain necessities was closing rapidly. 

"The first 72 hours are considered to be critical," said Steven Godby, a natural hazards expert at Nottingham Trent University in England. "The survival ratio on average within 24 hours is 74%, after 72 hours it is 22% and by the fifth day it is 6%."

The death toll keeps surging.

Turkish President Recep Tayyip Erdogan said Thursday that 14,014 people died in his nation, and more than 67,000 were injured. On the other side of the border, Syrian lost 3,577 people, and more than 6,300 were injured, bringing the death toll in the two countries to 18,000.

Risklayer, which describes itself as a "transparent and independent collaborative catastrophe risk firm in Germany and Australia," tweeted Wednesday that it projects the number of dead could blast above 45,000.