HARBIN -- Four shells suspected to have been left by the Japanese army during World War II have been unearthed in Qiqihar, a city in Northeast China's Heilongjiang province, local police said Friday.

The shells, each of which weighs 2.5 kilograms and is 30 centimeters long, were found by a group of workers on Sunday in a construction site near a railway station, according to the public security bureau of Ang'angxi district.

A worker spotted a rusty shell while digging about two meters deep into the ground. Later, the workers found another three shells nearby. They reported them to the police. People near the site were evacuated after the discovery, the bureau said.

The shells are believed to have been left by invading Japanese troops during World War II. They were well preserved and had a risk of explosion. The police have moved them to a safe place.

Two Japanese chemical weapon units were once stationed in Qiqihar. After the Japanese retreated, large amounts of bombs, shells and mines were left behind. Undiscovered explosives still pose a threat to local residents.