TOKYO: Four Chinese government ships spent several hours in territorial waters around disputed Tokyo-controlled islands on Thursday, for what Japan’s coastguard said was the first time in three weeks.

The move came as Tokyo and Beijing reportedly prepared for talks on a row that has derailed the relationship between Asia’s two largest economies and dented their huge trade ties.

It also came hours before outspoken nationalist Shintaro Ishihara—whose plan to buy the East China Sea islands sparked the last few months of hostilities—resigned as Tokyo governor to start his own national party.

Maritime surveillance vessels began entering the 12-nautical-mile zone around one of the islands early Wednesday, the Japanese coastguard said in a statement.

They remained there for more than seven hours before moving out to so-called contiguous waters, a band that stretches a further 12 nautical miles from shore.

Fisheries patrol ships and another vessel were spotted in waters near the archipelago, known as Senkaku in Japan and Diaoyu in China, coastguards said.

Maritime surveillance vessels and fisheries patrol ships are operated by different Chinese government agencies but are not military.

Japanese Vice Foreign Minister Chikao Kawai “strongly protested to the Chinese ambassador by telephone about the Chinese ships’ intrusion into Japan’s territorial waters”, the foreign ministry in Tokyo said in a statement.

In Beijing, foreign ministry spokesman Hong Lei said there was nothing abnormal about Chinese ships exercising jurisdiction in the area.

“The Chinese maritime surveillance vessels conducted routine patrols in the territorial waters around China’s Diaoyu Islands to safeguard the country’s sovereignty on October 25,” he said, according to the official Xinhua news agency.

Tensions have risen in recent months over the islands, which lie in rich fishing grounds. The seabed in the area is also believed to harbor mineral reserves.