Sri Lanka's central bank surprised financial markets on Tuesday by leaving its key policy rates unchanged, despite heavy pressure on the rupee currency and foreign outflows from government securities.

The central bank on Tuesday said it purchased 4 million dollars from the market on the previous day, but it has sold a net 184 million dollars to the market so far this year to defend the currency.

On Friday, the rupee touched its all-time low of 169.40 per dollar on importer demand for the greenback and foreign selling in government securities, but intervention by the central bank helped the currency close firmer, market sources said.

The rupee ended at 169.20/45 per dollar, compared with the previous close of 168.90/169.00.

The rupee weakened 4.7 percent in September against the dollar after a 1.2 percent drop in the previous month, and has declined 10.3 percent so far this year.

The Colombo stock index fell 0.17 percent to 5,807.74, its lowest close since Dec. 13, 2013. The bourse fell 3.6 percent last month and is down 8.8 percent so far this year.

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Data from the central bank showed foreign investors sold government securities worth a net 10.2 billion rupees ($60.36 million) in the week ended Sept. 26, the highest since the week to Dec. 6. Sri Lanka has seen a net outflow of 72.5 billion rupees in securities so far this year.

Stock market turnover was 395.2 million rupees ($2.34 million) on Tuesday, less than a half of this year's daily average of 784.4 million rupees.

Foreign investors sold a net 133.7 million rupees worth of shares on Tuesday, extending the year-to-date net foreign outflow to 6.01 billion rupees worth of equities.