Prime Minister Fumio Kishida has been quick to tighten Japan's border control over the Omicron variant of the coronavirus, imposing a ban on new entries by all foreign visitors from Tuesday.

For Kishida, who took office on Oct. 4 amid a lull in COVID-19 cases in Japan, the new variant is his first major test of leadership and he is not taking any risks by emulating his predecessor Yoshihide Suga, who was forced to resign amid criticism over his slow response and focus on economic activities.

"I am ready to face criticism that I am being too cautious," Kishida told reporters Monday, apparently expecting a backlash from the business community.

Japanese Prime Minister Fumio Kishida speaks to reporters at his office in Tokyo on Nov. 29, 2021, about border controls to keep out the new Omicron variant of the coronavirus. (Kyodo) ==Kyodo

While the characteristics of the variant and the effectiveness of vaccines for the strain remain unknown, Kishida said, "When dealing with an unknown risk, it's best to take every precaution."

His resolve came to a test on Tuesday when a government source said Japan has confirmed the first case in the country of the Omicron variant carried by a man in his 30s who arrived at Narita airport near Tokyo from Namibia on Sunday.

Under the new rules to be implemented for at least a month, Japan requires returning Japanese and foreign residents from 23 countries and regions to be quarantined at designated facilities as well as lowering its daily limit for the number of people arriving from 5,000 back down to 3,500.

Japan had the option of keeping the entry ban minimal by applying it only to South Africa and eight other African countries, but Kishida made the decision to expand its scope in the belief that infection controls must not be reactive, according to government sources.

The border closure comes at a time when Kishida's government is preparing to resume economic activities to help revive the country's pandemic-battered economy ahead of the House of Councillors election next summer.

An arrival lobby for international flights at Tokyo's Haneda airport is deserted on Nov. 30, 2021. Japan banned the same day new entries by foreigners globally for at least one month in an attempt to stave off the new Omicron variant of the coronavirus. (Kyodo) ==Kyodo

On Nov. 8, Japan eased its ban on new entries by foreigners, allowing businesspeople, students and participants in its technical internship program to enter on condition that their host organizations agreed to monitor their activities.

The Cabinet also endorsed last Friday a record 36.0 trillion yen ($314 billion) supplementary budget for fiscal 2021 to fund an economic stimulus package for easing the fallout from the pandemic.

There was also pressure from the ruling Liberal Democratic Party, with its policy chief Sanae Takaichi urging the Kishida government to take "measures based on the worst-case scenario."

The government's top spokesman, Chief Cabinet Secretary Hirokazu Matsuno, has echoed that view, repeatedly saying, "The essence of crisis management is to consider the worst-case scenario."

"We want to enter the upper house election (campaign) after giving a boost to the economy, but failure to deal with the Omicron variant could deal a fatal blow" to the party, a senior LDP member said.

Japan's enhanced border control is at the strictest level among countries that have taken steps against the spread of the Omicron variant, which the World Health Organization has designated a "variant of concern."

Britain has not totally banned new entries but requires virus testing for all people entering the country and called on people to self-quarantine regardless of whether they have been vaccinated if they are suspected to have come into contact with those infected with the new variant.

The United States restricts entry from South Africa and seven other African countries.

But whether Japan can prevent the variant from entering the country with the strict measures remains unknown, given the strain is rapidly spreading.

Before the confirmation of the first infection in Japan of the variant, a health ministry official admitted there is a possibility that people who came into close contact with the man from Namibia have already entered Japan.

"The infectiousness of the Omicron variant is believed to be strong, and there is a high likelihood of it entering Japan," said Satoshi Hiroi, a virology researcher at the Osaka Institute of Public Health.

Related coverage:

Japan confirms 1st case of Omicron variant

Japan bans foreign visitors for 1 month over Omicron fears

WHO says new Omicron coronavirus variant is "of concern"