Seoul expressed "extreme regret" Friday over Japan's ordering 14-day quarantines on all visitors from South Korea due to a surge in viral infections and warned of retaliation if Tokyo doesn't withdraw the restrictions.
The response came a day after Japanese Prime Minister Shinzo Abe announced the quarantine, which also applies to visitors from China.
"The Japanese government yesterday issued what's realistically a full entry ban on our people," South Korean Prime Minister Chung Se-kyun said during a government meeting on quarantine strategies. "(We) consider this as very regrettable and demand the excessive and irrational measure to be immediately withdrawn."
Japan's move could further strain bilateral relations between the Asian U.S. allies, which sank to their lowest point in decades last year as they feuded over trade issues, wartime history and military cooperation.
Chung Eui-yong, South Korea's presidential national security director, presided over a national security council meeting where officials criticized Japan for enforcing the travel restrictions without consulting with Seoul first and agreed to review countermeasures.
The officials said South Korea's active quarantine efforts were in contrast to what is said was Japan's "non-transparent and passive" approach to its own outbreak, the Blue House said in a statement.
South Korea's Foreign Ministry said its first vice minister, Choi Se-young, plans to summon Japanese Ambassador Koji Tomita on Friday to protest Japan's move.
In a text message sent to reporters, the ministry questioned whether Japan's entry restrictions were politically motivated. It insisted that South Korea has started to see progress from its "transparent and active" quarantine efforts to contain the illness, which has so far sickened more than 6,000 people, the most in any country outside of China.
The ministry said Seoul will consider "every possible" countermeasure, such as designating Japan as a risk area for infectious diseases and placing its own restrictions on travelers from Japan. However, South Korea has not taken any tit-for-tat steps against more than 90 other countries that banned or restricted visitors from the country.