Thai tourists complain that too many are being refused entry to South Korea as authorities there fear they may overstay their visas or look for work, but the Thai Foreign Ministry (MFA) has cited a lack of required documents and language problems as the main reasons.
Ministry of Foreign Affairs spokeswoman Busadee Santipitaks said tourists must be careful to prepare all the right documents for immigration officials.
Many Thais also struggle when it comes to communicating with South Korean officials, she said.
Ms Busadee was speaking after a Facebook post came to the fore showing that a Thai national had been singled out from her friends and refused entry by Korean immigration officers for no clear reason.
The Facebook user said she had subsequently learned many of her compatriots had a similar experience.
Thai tourists now enjoy a 90-day visa exemption to South Korea.
"A surge in the number of Thais living illegally in South Korea is the main cause of immigration authorities refusing entry to Thai tourists upon arrival," Ms Busadee said.
She said she had been informed of the problem by the Thai embassy in Seoul.
"Thais are often found to have violated South Korea's immigration laws, such as overstaying their visas or working illegally," she said.
In March there were 101,000 Thais living in South Korea. A total of 57,490 Thais were found to have overstayed their visas and are now residing in the country illegally, according to the MFA.
Roughly, 20,000 Thais are estimated to be denied entry every year, Ms Busadee said.
To avoid eing detained or deported upon arrival, Ms Busadee asked Thai tourists to prepare all of the necessary papers including round-trip air tickets, travel itineraries, hotel reservations, contact details and financial statements showing they have enough money to spend during their stay in South Korea.
Ms Busadee has pledged to solve the problem so Thais wishing to visit the East Asian nation will not encounter any difficulties.
But tourists must respect a foreign country's immigration regulations, she added.
The Royal Thai embassy in Seoul earlier warned that those who are caught working illegally will be deported, blacklisted or banned from visiting South Korea for up to five years.