Police conducted a joint drill with the Japan Coast Guard at a port in Kanazawa, Ishikawa Prefecture, on Thursday to combat attempts being made to illegally enter Japan, officials said.
Deploying helicopters and patrol boats, about 200 officers from the police and coast guard, as well as from the local customs and immigration offices, took part in the operation on the Sea of Japan coast.
The drill was based on the assumption that illegal immigrants were trying to enter Japan aboard a small boat to which they had transferred from a foreign freighter off the port, the officials said.
"We hope to sharpen our cooperation so that we will be better prepared for cases of illegal immigration, which are on the increase," said Susumu Takahata, chief of the Kanazawa West Police Station.
Local authorities have been on the alert for illegal entries ever since police found a jet ski in late March at the mouth of a river in neighboring Toyama Prefecture.
The jet ski is similar to one found in 1990 farther along the coast in Mihama, Fukui Prefecture, near the body of what Japanese authorities believe was a North Korean agent.
The government estimates there are 250,000 foreigners illegally staying in Japan, some of whom entered the country illegally and others who have overstayed their visas.