SEOUL (Reuters) - South Korea imposed unilateral sanctions on 18 North Koreans on Monday, barring any financial transactions between those sanctioned and any South Koreans, as part of international efforts to dry up Pyongyang s illegal cash flows. All 18 individuals on which the South Korean sanctions were imposed were directly affiliated to North Korean banks, according to an official government announcement by the finance minister uploaded on the Interior Ministry s website.  They are all people at North Korean financial institutions that have already been sanctioned by the United Nations,  a government official directly involved in the sanction development process told Reuters. The official asked for anonymity as he was not authorized to speak to media.   They re high-ranking employees who have been linked to North Korea s nuclear and missile development program as well as the North s foreign exchange procurement efforts.   We aim to encourage people to avoid problematic transactions with North Korea (through these sanctions) and to exercise caution with North Korea-related transactions in general,  said Unification Ministry spokesman Baek Tae-hyun in a regular media briefing.  We expect this will contribute to blocking North Korea s main sources of foreign exchange and its developing of weapons of mass destruction.  The announcement came a day ahead of U.S. President Donald Trump s visit to South Korea as part of his 12-day Asia tour, where he is expected to discuss North Korea s nuclear and missile program at length with government officials here.  When asked on the timing of the announcement, government officials denied it was tied to Trump s visit to Seoul. Despite the announcement, the sanctions are expected to have little impact on North Korea s illegal activities to fund its weapons program as all trade and financial exchanges have been barred since May 2010 following the torpedoing of a South Korean warship by North Korea. Those named were: Kang Min, Ko Chol Man, Ku Ja Hyong, Kim Kyong Il, Kim Tong Chol, Kim Sang Ho, Kim Jong Man, Kim Hyok Chol, Ri Un Song, Ri Chun Song, Ri Chun Hwan, Mun Kyong Hwan, Park Mun Il, Pak Bong Nam, Pang Su Nam, Pae Won Uk, Chu Hyok and Choe Sok Min. According to the announcement 14 of those named were based in China, two were based in Libya while the remaining two were based in Russia. The institutions the 18 were affiliated with were Korea Daesong Bank, Foreign Trade Bank of the Democratic People s Republic of Korea, Korea United Development Bank, Bank of East Land and ILSIM International Bank.  As of the announcement on Monday, South Korea had North Korea sanctions imposed on 97 individuals. 