NHK has learned of extensive damage to the eastern Ukrainian city of Mariupol after analyzing satellite imagery with AI technology.

Mariupol is said to be the fiercest battleground of the Russian invasion. Residents were killed in attacks on a theater and a hospital there.

Mariupol Mayor Vadym Boichenko said on Monday that more than 20,000 people have died. But the entire picture of damage to the city remained unclear.

An NHK team obtained high-resolution images of central Mariupol taken on April 3 from US satellite operator Planet. The team compared images of a roughly 5-square-kilometer area of the city with those from August 2021, with help from experts in AI imagery analysis.

The team divided the area into 20-square-meter segments. Those where more than 5 percent of buildings were found to have changed were marked in yellow. The higher the percentage of structural alteration, the closer the color turned to red.

As a result, the entire 5-square-kilometer area was covered in red or yellow.

A closer look at places marked red shows massive damage to a large shopping mall and an arts college in the north, as well as many homes in the west with destroyed roofs.

The actual damage may extend over a wider area, as the method of analysis has a margin of error due to clouds and shadows. In addition, traces of attacks on sides of buildings are hard to detect from above.

Ukrainian authorities have described the destruction in Mariupol as a war crime, but Russia has repeatedly claimed that it targeted only military facilities.

Asada Masahiko, a professor at Doshisha University and an expert on international law, expressed shock at the extent of destruction. He indicated that homes and schools were very likely attacked intentionally and indiscriminately in violation of international law.