The association, known as Vicofa, said in a statement that the dry weather spell was "quite severe" in the main coffee-growing area in the Central Highlands, where it hasn't rained since the end of November. Vicofa said 40% of the area in the region is at risk of water shortages.

A spell of dry weather is affecting much of Southeast Asia, affecting the production of another key commodity produced in the region: palm oil. February was the driest month on record in Singapore, while in Malaysia, authorities in Selangor province have started rationing water.

Cold weather and frost also affected coffee trees in the Central Highlands and Son La province in northwest Vietnam, said Vicofa. It said over 1,300 hectares of higher-quality arabica coffee had been affected in Son La.

Vietnam is the world's largest grower of robusta coffee, a variety used mainly in instant-coffee blends because of its more bitter taste.

But a drought in Brazil, the biggest grower and exporter of arabica beans, has lifted demand for robusta as some roasters choose to use cheaper and lower-quality beans in their blends.

Prices for robusta coffee futures have risen more than 20% this year. On Thursday, the Liffe March contract for robusta was up 2.3% at $2,161 a ton.

Abah Ofon, a commodities analyst at Standard Chartered PLC in Singapore, said the "big story" in coffee this year was the weather in Vietnam's Central Highlands. "If robusta output is constrained, that's going to contribute to more a bullish environment for the entire coffee complex," he said.

The supply of robusta beans already has been tight this year. Vietnamese growers began withholding stocks late last year due to low prices for physical beans and are still holding out for higher prices.

Trang Quang Thanh, deputy director of the Agriculture and Rural Development Department of Dak Lak province, the largest coffee-growing area in the Central Highlands, was more positive on the outlook for the crop in his region. He said the coffee farms have wells and pumping systems to water the trees.

"But if the dry season is longer than usual, the underground water level will be too low, then there will be a problem," he added. "The coffee trees here have blossomed well this year, though."

By Isabella Steger & Vu Trong-Khanh - The Wall Street Journal - March 6, 2014