The Kyushu region of southwestern Japan has suffered more than 100 aftershocks since it was hit by a 6.8 magnitude earthquake on Tuesday that left at least 13 dead.

Many others were said to be in “cardiopulmonary arrest,” a term often used in preliminary reports in Japan before a death is officially confirmed.

Amid scorching heat, rescue teams are digging through the rubble of a paper factory and a shopping mall in Kumamoto Prefecture, searching for trapped survivors. The shopping center suffered an explosion an hour after the earthquake.

Japanese Prime Minister Sanae Takaichi warned that it was “truly a race against time” to rescue those trapped. Thousands of people are in evacuation centers and many of them are without electricity.

The Japan Meteorological Agency (JMA) is urging residents to be alert to the possibility of more strong earthquakes, public broadcaster NHK reported.

“Over the next week, especially over the next two or three days, we would like people to be on the lookout for possible earthquakes with a maximum seismic intensity of 7,” a JMA official said.

Rescuers continue to search through the rubble of the Aeon shopping center in the city of Kashima, where part of the second floor collapsed.