COLOMBO Aug 18 (Reuters) - Sri Lankan shares posted a three-month closing high on Thursday, led by gains in blue chips such as John Keells Holdings Plc and Ceylon Tobacco Company Plc, while foreign investor buying also boosted sentiment.

The benchmark Colombo stock index ended 0.23 percent, or 15.30 points, firmer at 6,593.25, its highest close since May 20.

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Foreign investors, who have been net sellers of 3 billion rupees ($20.63 million) of shares this year, bought a net 343.2 million rupees in their highest purchase since Aug. 1, extending the net foreign inflow in the past 16 sessions to 1.8 billion rupees worth of equities.

"Investors are more focused on blue chips such as John Keells," said Dimantha Mathew, head of research at First Capital Equities (Pvt) Ltd, adding profit-taking had dried up.

Turnover got a boost as a foreign investor sold shares of National Development Bank Plc to other foreign investors, Mathew said.

Turnover stood at 1.78 billion rupees, well above this year's daily average of around 739.1 million rupees.

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Shares of John Keells Holdings rose 0.96 percent, while Ceylon Tobacco Company gained 1.09 percent.

Commercial Bank of Ceylon Plc, the country's biggest listed lender, rose 1.46 percent, while National Development Bank ended 0.12 percent weaker. ($1 = 145.4000 Sri Lankan rupees) (Reporting by Ranga Sirilal and Shihar Aneez; Editing by Subhranshu Sahu)