Taiwanese authorities have arrested two Chinese nationals who sailed over in a small boat and attempted to land on an island.

It is the third incident this month in which Chinese nationals appear to have reached Taiwan by boat, stoking security concerns.

Taiwan has accused Chinese authorities of using these incidents to “harass” them. Beijing has yet to respond, nor has it commented on the recent intrusions.

Taiwan considers itself a sovereign country, but China views it as one of its provinces and has not ruled out the possibility of using force to claim it.

The Taiwanese coastguard said in a statement they had detected the two men sailing into their waters on a sampan, or a small wooden boat, in the early hours of Tuesday.

The men were caught “launching a styrofoam float” from their Chinese-registered sampan in the waters off Erdan Island.

Pictures released by the coastguard appear to show a man kneeling on a small floating board with a paddle in his hands.

Erdan is part of Taiwan’s Kinmen district, which is a mere 5km (3 miles) from the Chinese coast.

Taiwan was on high alert for possible threats from China on the day the men were apprehended: the one-year anniversary of the inauguration of Taiwanese president William Lai, whom Beijing reviles.

Last week a Chinese father and his teenage son landed in Taoyuan on the northern tip of Taiwan’s main island, after crossing over in a small rubber dinghy.

Questions were raised about how they managed to travel more than 100km across the Taiwan Strait – one of the most heavily patrolled patches in the region and a busy international shipping lane – without being discovered.

The coastguard confirmed the landing occurred on 16 May, but admitted that they had not detected the boat before it landed as it was too small to be picked up on their radar technology.

The father and son, who were from the southern Chinese province of Fujian, reportedly claimed they were victims of persecution in China and wanted to “seek freedom” in Taiwan.

In another case that has stoked worries, a Chinese social media influencer posted several clips online appearing to show that he had travelled across the Taiwan Strait and landed on a beach, also in Taoyuan.

The influencer, who goes by the nickname Shandong Kaige, claimed he had set off from Fujian on 15 May and travelled for about nine hours on a motorised rubber dinghy.

He was seen next to a Chinese flag on a beach, which he claimed he had planted “on this territory so beloved by the motherland”. He also said he returned to China on the same day.

Taiwanese authorities have analysed the footage and concluded that it was authentic. They confirmed the man was in Taoyuan, but said they were still investigating whether the man had travelled by boat across the strait or had flown into Taiwan.

Anyone caught illegally entering Taiwan can face up to five years in jail and a fine of up to 500,000 Taiwanese dollars (£12,300; $16,600).

Taiwan used to see thousands of Chinese attempting to enter illegally every year, particularly in the 1990s when Taiwan’s booming economy attracted impoverished Chinese looking for work.

While overall numbers are much lower now, there appears to be an uptick in recent years, according to official figures published in Taiwanese media.

There were 15 Chinese caught in 2023, and while there are no available figures for the whole of last year, 23 were caught between June and September.

Since January this year the coastguard has caught about 40 people including the men in Tuesday’s case – although it is not known how many of those were from China.

On Tuesday, the coastguard said there has been a “recent surge in the number of cases in which the Chinese Communist Party has used small targets to harass Taiwan” as part of its greyzone warfare.

Taiwanese authorities have not ruled out that Beijing would “use illegal immigration by boats to conduct cognitive warfare”, the coastguard added.

It has also said there is an “urgent need” to upgrade its surveillance technology to combat these threats.

Analysts say that China has been using greyzone tactics, which are manoeuvres that fall short of an invasion and exist in a grey area, to weaken Taiwan over a prolonged period.

These include Chinese warplanes and coastguard boats repeatedly entering Taiwan’s airspace and waters.

More recently, Taiwanese authorities have alleged that Chinese-linked commercial ships have been deliberately damaging Taiwan’s undersea communication cables.

Additional reporting by Ian Tang.