* Lockdown near Bangkok sparks sell-off in Thai markets * New strain of coronavirus in UK dents sentiment * Seoul imposes harsh social distancing measures, won drops By Anushka Trivedi Dec 21 (Reuters) - The Thai baht was set for its worst day in more than three months on Monday and stocks sold off heavily after coronavirus curbs in a key export hub near Bangkok were extended following a record number of infections in the country. A seafood industry centre that is home to thousands of migrant workers and is economically crucial to Thailand, Samut Sakhon will stay locked down until Jan. 3 after an outbreak at a shrimp market over the weekend added over 500 cases to the tally. Previously among the biggest beneficiaries of the vaccine news, Thai equities slid 3% and the baht fell almost 1% on Monday to lead declines in Asian emerging markets due to the economy"s heavy dependence on trade and tourism. "In relation to the Samut Sakhon spread, not just for the region, but there is risk of a nationwide lockdown, which represents further economic pressure and thus had been a hit to market confidence and prices alike," said Jingyi Pan, a senior market strategist at IG in Singapore. However, the country"s prime minister Prayuth Chan-ocha called for calm and said there were no immediate plans for a wider lockdown and the government would monitor the situation for a week before taking action. The surge in cases comes as Thailand attempts to revive its COVID-19 pandemic-ravaged economy by softening some rules on tourism by allowing more foreign visitors to return. Meanwhile, the South Korean won dropped 0.3% after Seoul imposed stringent social distancing measures in the city as the island-nation battles its worst coronavirus outbreak yet. Other Asian currencies buckled as investors sought the safety of the dollar on news that a new coronavirus strain had led to a tougher lockdown in the United Kingdom. The Indonesian rupiah and the Singapore dollar fell about 0.2% and 0.6%, respectively, while the Taiwanese dollar was the only exception, moving up 1.2%. Most Asian equity markets slipped, including Manila and Kuala Lumpur, although China shares added 0.8% on more policy affirmation from Beijing and helped cap losses in the regional benchmarks. HIGHLIGHTS ** Thailand"s 10-year government bond yields are up about 4.30 basis points at 1.22% ** Top losers on Thailand"s SETI include Nok Airlines PCL down 14.67% and Nirvana Daii PCL down 12.9% ** Top gainers on the Jakarta stock index include Lotte Chemical Titan Tbk PT up 34.6% and Bank Pembangunan Daerah Banten Tbk PT up 34.3% Asia stock indexes and currencies at 0746 GMT COUNTRY FX RIC FX DAILY % FX YTD % INDEX STOCKS DAILY % STOCKS YTD % Japan -0.24 +4.88 -0.18 12.93 China -0.19 +6.29 0.76 12.15 India -0.24 -3.19 -0.76 12.22 Indonesi -0.28 -1.70 1.12 -2.02 a Malaysia -0.26 +1.00 -0.46 3.54 Philippi +0.02 +5.39 -0.66 -7.55 nes S.Korea -0.27 +4.87 0.23 26.44 Singapor -0.63 +0.68 -0.01 -11.61 e Taiwan +1.24 +6.99 0.95 19.90 Thailand -0.80 -0.40 -3.36 -9.33 (Reporting by Anushka Trivedi in Bengaluru; Editing by Bernard Orr)