Traders forecast the output to be around 27 million 60-kg bags for the crop year starting October 1, down from their previous outlook of around 30 million bags.

"The beans are smaller than previously thought because the rainy season ended too early this year," a trader in Dak Lak province in Vietnam's Central Highlands told Reuters.

"Normally the rain continues through September and October, but it didn't this year, resulting in smaller beans," the trader said.

Farmers in the Central Highlands, Vietnam's largest coffee growing area, said the harvest has started there but it won't peak until early next month.

Farmers in the Central Highlands sold coffee at 37,000 dong ($1.59) per kg COFVN-DAK, up from 36,300 dong a week earlier, traders said.

"Trade is slow as inventory has run empty while fresh beans of the new harvest are not yet ready for shipping," another trader based in Ho Chi Minh City said, adding that more beans of the new harvest will be available for exports from late this month.

Traders in Vietnam offered the 5-percent black and broken grade 2 robusta COFVN-G25-SAI at $90-$100 discount per tonne to the January contract, widening from $50-$90 last week.

In Indonesia, Robusta beans of Lampung were traded at a $30 premium to January contract this week, compared with a range of $20-$30 premium last week, an exporter in Bandar Lampung said.

"Trade is quiet this week due to low supply," the trader said, noting that not much demand was seen in Indonesian market this week.