That was almost half the 4.1 million visitors from the mainland in 2015 under the Beijing-friendly administration of president Ma Ying-jeou but the island’s tourist numbers were bolstered by a sharp increase in visitors from countries targeted by the “New Southbound Policy” adopted by Taiwanese President Tsai Ing-wen of the independence-leaning Democratic Progressive Party.

The policy aims to shift the island’s heavy trade reliance on the mainland to 18 countries, including those in Southeast Asia, India, Australia and New Zealand.

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The bureau said more than 2.3 million visitors from the policy’s targeted countries visited the island this year – up more than 15 per cent from last year. In all, 1.49 million people visited from Hong Kong, 1.77 million from Japan, 910,175 from South Korea, and 518,154 from the United States.

The number of visitors from the mainland has plunged as cross-strait ties have declined in the last few years, with Beijing’s insistence that Tsai acknowledge the one-China principle, something she has not been willing to do.

A key part of Taiwan’s new regional push is the Kuan Hung Pilot Project, an electronic visa programme launched in 2015 lowering visa requirements for tourists from Cambodia, India, Indonesia, Laos, Myanmar and Vietnam. The programme allows tourists from the six countries to apply for Taiwanese visas free online without proof of income.

However, Taiwanese authorities announced a review of the five-year project following the disappearance of 152 Vietnamese who entered the island on four tour groups on December 21 and December 23.

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The visitors slipped away at night from their tour groups in Kaohsiung, southern Taiwan, with 127 still unaccounted for, according to the National Immigration Agency.

Authorities fear the missing tourists might have entered the island to work illegally.

Taipei said on Monday that the authorities would tighten entry to Taiwan immediately.

“This includes more checks and questioning at immigration points, such as whether a visitor from Vietnam has a return plane ticket and accommodation in Taiwan,” Cabinet spokeswoman Kolas Yotaka said.

Group tour applications would also come under greater scrutiny, visas would be revoked if there were irregularities and travel agencies would be held responsible for the group and for telling authorities about any violations by visitors, she said.

Since 2015, 225,702 tourists from the six countries have visited the island, with 566 going missing, according to the NIA. All but five of the missing were Vietnamese.

That compares to 903 unaccounted-for visitors among the 19.8 million mainland tourists since Taipei allowed cross-strait arrivals in 2008.

Travel agents said the smaller proportion of missing tourists from the mainland was due to fines of up to NT$1 million (US$32,650) facing agents if visitors were unaccounted for.

Police in Kaohsiung said one of the 21 tourists found since going missing in Kaohsiung claimed that he had been told in Vietnam that he could find a job in Taiwan. He said he paid US$1,000 to join the tour group to Kaohsiung, where he and other members of the group were picked up and driven to Tainan.

Liu Jui-fu, head of an NIA task force based in New Taipei, said he suspected an illegal ring was matching Vietnamese with illegal jobs on the island.

In a separate case, police in Taichung, central Taiwan, arrested 16 Vietnamese over an alleged telecom fraud to dupe people in Vietnam out of money. The Criminal Investigation Bureau accused the detainees of scamming about US$490,700 from 200 victims in Vietnam over the past three months after entering Taiwan on the Kuan Hung programme.