Singapore shares fell by midday after the previous day's rally, but commodities firm Olam International Ltd bucked the weak market, extending its gain since launching its first share buyback.

The Straits Times Index was down 0.6 percent at 2,770.35 points, while MSCI's broadest index of Asia-Pacific shares outside Japan slipped 0.7 percent. The Singapore bourse rose 1.8 percent on Monday.

Some of the biggest decliners on the index were Global Logistic Properties, which fell more than 3 percent, and Sembcorp Marine, losing 1.6 percent.

Olam shares rose as much as 2.5 percent, bringing its total gain since its share buyback announcement late on Friday last week to nearly 12 percent. Analysts said the move signals the management's confidence in the firm's long-term prospects.

But even after the rally, Olam stock was still down about 15 percent so far this year, underperforming the nearly 5 percent gain in the broader index.

Citigroup said Olam's price-earnings ratio valuation for equity is at similar lows to those seen during the global financial crisis, as the commodity sector was hit and it will take longer for growth to come from some of its investments.

Olam has a lower gearing in this cycle, with adjusted net gearing of 0.4 times, leaving it room to fund further debt or equity buybacks as it had raised S$740 million ($576.9 million) in new equity last year, Citi said.

1135 (0335 GMT) (Reporting by Eveline Danubrata in Singapore; eveline.danubrata@thomsonreuters.com)