TOKYO (Reuters) - Japan said on Monday it would respond firmly after Chinese government vessels intruded into what Japan considers its territorial waters near disputed islands in the East China Sea 14 times at the weekend.

Ties between China and Japan, the world’s second and third largest economies, have for years been plagued by a dispute over the islands that Japan controls, and the waters around them.

The flurry of Chinese incursions into the waters follows a period of sustained pressure on China about its activities in the South China Sea, and a Chinese criticism of what it saw as Japanese interference in that dispute.

Chinese activity near the disputed East China Sea islands, known as the Senkaku in Japan and the Diaoyu in China, has heated up since Friday, Japanese officials said, prompting repeated Japanese protests, including three on Sunday alone.

Japan’s Chief Cabinet Secretary Yoshihide Suga said Japan would urge China not to escalate the East China Sea dispute, while also responding firmly and calmly.

Agencies including the coastguard would act closely together to deal with the situation, Suga said.

A Japanese government source, who asked not to be identified, said Japan’s coastguard had stepped up its patrols in the region at the weekend but declined to give further details.

Japan's Chief Cabinet Secretary Yoshihide Suga (C) arrives at Prime Minister Shinzo Abe's official residence in Tokyo, Japan, July 2, 2016, following news of an attack on a restaurant in the Bangladeshi capital by gunmen. REUTERS/Toru Hanai

About 230 Chinese fishing vessels were in the area on Saturday, Japan’s foreign ministry said.

China’s foreign ministry spokeswoman Hua Chunying said in a statement on Saturday that China had indisputable sovereignty over the islands and nearby waters.

In the South China Sea, Japan has no claims and China recently rejected warned Japan not to interfere.

The United States, its Southeast Asian allies and Japan have questioned Chinese land reclamation on disputed islands in the South China Sea, especially after an international court last month rejected China’s historic claims to most of that sea.

China has refused to recognize the court ruling. Japan called on China to adhere to it, saying it was binding. China warned Japan not to interfere.

The spike in tension over the East China Sea also follows a Chinese accusation that Japan’s new defense minister, Tomomi Inada, had recklessly misrepresented history after she declined to say after her appointment last week if Japanese troops had massacred civilians in China during World War Two.

The legacy of Japan’s wartime occupation of parts of China is another thorn in relations between the neighbors.

China, and other counties in Asia, in particular South Korea, feel that Japan has never properly atoned for its aggression before and during World War Two.

Relations between South Korea and China have also been strained in recent days by a decision by South Korea and the United States to deploy an advanced anti-missile defense system, to guard against North Korean attacks, that China fears could be used against its military.

South Korea’s presidential office on Sunday rebuked China over its criticism of South Korea’s decision to deploy the anti-missile defense, urging China instead to play a stronger role against North Korea’s provocations.

South Korea and the United States began discussions to deploy the Terminal High Altitude Area Defence (THAAD) unit in the South after the North’s fourth nuclear test in January and a long-range rocket launch in defiance of U.N. sanctions.