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By Sameer Manekar

March 28 (Reuters) - South Korean and Thailand equities tumbled on Friday after new U.S. tariffs on auto imports jolted car and auto parts makers, while participants in Asian emerging markets braced for the reciprocal U.S. tariffs set to take effect next week.

Thailand's benchmark index .SETI slid 1% to a one-week low and the baht THB=TH weakened 0.4%, as Southeast Asia's biggest auto production centre and an export base for top car makers stood exposed to the new U.S. auto tariffs.

"The U.S.'s higher import tariffs on automobiles and auto parts will weigh further on Thailand's ailing economy via an additional drag on the automotive sector," said Charnon Boonnuch, an ASEAN economist at Nomura, noting that the reciprocal and auto sector tariffs announced were a downside risk to Thailand's growth outlook.

South Korea's KOSPI .KS11 toppled nearly 2% in its second straight day of losses, dragged lower by sharp losses in Hyundai Motor 005380.KS and sister automaker Kia Corp 000270.KS.

The index has lost more than 3% over the past two sessions as the two automakers lost billions of dollars in value.

"Autos account for 9.4% of Asia's exports to the U.S., but the exposure is highest for South Korea and Japan," said Sonal Varma, Chief Economist for India and Asia excluding Japan at Nomura.

Taiwan's benchmark index .TWII also slid nearly 2% to its weakest level since mid-September last year. TSMC 2330.TW, the world's top contract chipmaker, plumbed to its mid-September low point.

The Korean won KRW=KFTC and Taiwan's dollar TWD=TP slipped around 0.3% each.

An MSCI gauge of Asia EM equities .MIMS00000PUS skidded 0.8%, while a broader gauge of world EM equities .MSCIEF was down 0.7%.

While analysts anticipate limited direct effects from U.S. tariffs on trade-reliant Southeast Asian economies, they remain vulnerable to reciprocal U.S. tariffs and slower growth in the global economy and key trading partners.

Singapore's Straits Times index $(STI)$ .STI breached the 4,000 level for the first time, led higher by a rally in top banks.

This is a "key psychological milestone, which could be interpreted as strong investor confidence, resilient blue-chip performance, and a favourable economic climate," said Daphne Tan, Director of Business Development at CMC Markets Singapore.

"Breaking this level suggests a strengthening sentiment, but sustaining it would depend on global stability and market fundamentals."

Malaysia's ringgit .MYR=, Singapore's dollar SGD=, and the Philippine peso PHP= edged lower. India's rupee INR=IN appreciated 0.15%, while its Nifty 50 index .NSEI was largely unchanged in early trade.

Indonesia's financial markets IDR=, .JKSE remain closed for a national holiday till April 7.

HIGHLIGHTS:

** Trump Organization eyes multi-billion-dollar projects in Vietnam amid tariff risks

** Philippines' trade deficit narrows to $3.15 bln in Feb

** Indonesia's Widjaja family offers to take Sinarmas Land private in $986 million deal

** Malaysia's 10-year bond yield MY10YT=RR hovers at 3-week high of 3.781%

(Reporting by Sameer Manekar in Bengaluru; Additional reporting by Ankur Banerjee in Singapore; Editing by Janane Venkatraman

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((Sameer.Manekar@thomsonreuters.com;))