China is stepping up patrols in the waters off the coast of Taiwan’s Kinmen archipelago, days after two of its fishermen drowned while being chased by the Taiwanese coast guard, which accused the boat of trespassing.

In this handout photograph provided by Taiwan Coast Guard Administration, Taiwanese coast guards inspect a vessel that capsized during a chase off the coast of Kinmen archipelago in Taiwan, Wednesday, Feb. 14, 2024. The unnamed vessel had been sailing about one nautical mile off the coast of an islet of Kinmen island and was trespassing, said Taiwan’s Coast Guard Administration. Four fishermen fell into the water and two survived and are in “good shape,” while resuscitation efforts on the other two failed, the vice director of the Coast Guard in Kinmen, Chen Jien-wen, told a local TV channel.

The Chinese coast guard's Fujian division will regularly monitor the waters off the southern coast of the city of Xiamen — a few kilometers from Kinmen — to strengthen maritime law enforcement, said the coast guard's spokesman, Gan Yu, in a statement Sunday.

Fishermen from Taiwan and China regularly sail that stretch of water which has seen a rise in tensions as the number of Chinese vessels — including sand dredgers and fishing boats — have notably increased in the area.

Kinmen residents have complained of both the noise and sound pollution from the vessels, as well as losses to their livelihood in fishing.

The fishermen’s deaths are unusual despite the level of Chinese activity in the waters near Kinmen, which is closer to China than it is to Taiwan’s main island. China claims all of self-ruled Taiwan as part of its territory.

On Wednesday, Taiwan's Coast Guard said two of four Chinese fishermen died after their boat capsized. It said their boat was fishing about one nautical mile away from Kinmen archipelago which Taiwan has claimed as a restricted area. The other two survivors remain in Taiwan's custody.

China has issued a furious condemnation and blamed Taiwan's ruling Democratic Progressive Party for the fishermen’s deaths. It also said that there was no such thing as “restricted” waters.

Taiwan said an investigation was underway and that its Mainland Affairs Council said they were communicating with Chinese authorities.

China's Taiwan Affairs Office vowed on Sunday further measures, without clarifying further detail.

“The Mainland reserves the right to take further action, and all the consequences will be borne by Taiwan,” said Zhu Fenglian, spokesperson for the Taiwan Affairs Office.

Taiwan split from China during the 1949 civil war, but Beijing continues to regard the island of 23 million with its high-tech economy as Chinese territory and has been ramping up its threat to achieve that by military force if necessary.