Seoul, Sep 1 : One in five foreign nationals staying in South Korea are illegal immigrants, with the number of undocumented immigrants rising to nearly 400,000, government data revealed on Thursday. According to the data compiled by the Ministry of Justice, 395,068 foreigners were staying in the country without a valid visa in July, about 20 per cent of the entire population of foreign nationals in South Korea, reports Yonhap News Agency. The number of undocumented foreign nationals has steadily increased, from 388,000 last December to some 392,000 in March and some 394,000 in May, the data showed. The ratio of undocumented foreigners out of the entire population of foreign nationals also jumped from 15.5 per cent in 2019 to 19.3 per cent in 2020 and 19.9 per cent in 2021, the data showed. The Ministry attributed such increase to more foreigners not returning to their home country due to tightened border controls following the outbreak of the Covid-19 pandemic. /IANS