By Ben Otto and Joko Hariyanto

JAKARTA, Indonesia--Indonesia booked a wider-than-expected trade deficit of $270 million in September, partly because of weakening investment, data showed Monday.

In September, exports from Southeast Asia's largest economy totaled $15.28 billion, up 3.9% from a year ago, while imports notched $15.55 billion, up 0.2%, Indonesia' official statistics agency said. Through September, Indonesia has posted a $1.66 billion trade deficit in 2014.

A survey of economists polled by The Wall Street Journal forecast a narrowing monthly deficit of $135 million, from a revised $312 million in August, in light of resumed copper-concentrate exports by the local units of Freeport-McMoRan Copper & Gold and Newmont Mining Corp. Earlier this year, Indonesian regulators forced miners to refine minerals domestically, which halted exports for several major miners until they recently agreed to pay new export taxes.

Capital-goods imports also remained sluggish, trending at a 7% decline for the year, according to DBS Bank Ltd. Foreign investment in Indonesia is at a record high, but the pace of investment has slowed over the past year as economic growth has slipped to just more than 5% from almost 6% in 2013.

"At the very least, imports and exports are not falling anymore," said Gundy Cahyadi, economist for DBS. "The economy needs strong domestic demand to continue its medium-term expansion."

Write to Ben Otto at ben.otto@wsj.com

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