U.S. soybean and soymeal futures extended gains on Thursday to trade at levels last seen in 2014, as investors continued to fret about the prospect of reduced supply from exporter Argentina after a rain-hit harvest.

Soy and grains also found support in rallying oil prices, with Brent futures reaching $50 for the first time in seven months, and a slight fall in the dollar.

Corn reached its highest since last July while wheat rose for a third day, helped by potential damage to maturing U.S. winter wheat due to wet weather.

The Chicago Board of Trade most-active soybean contract were up 1.1 percent at $10.97-1/2 a bushel by 1122 GMT, a highest level for a most-active contract since late July 2014.

Soymeal added as much as 2.7 percent to $418.3 a short ton, setting an 18-month high for the second straight session.

Soymeal, which surged 4.7 percent on Wednesday, has led the rally in the soy complex in recent weeks as the market has reacted to news of weather damage in Argentina, the world's largest soymeal exporter.

Flooding is expected to have cut soybean output by between 4 million and 8 million tonnes, even as higher-than-expected yields in drier areas offset some of the losses, local farm analysts said on Wednesday.

Soybean exports could fall by as much as 25 percent this year. However, some analysts say higher soy prices will prompt U.S. farmers to plant more of the oilseed than initially expected this spring, which could cool the market rally.

"The main concern driving soybean and meal prices higher is the damage to crops in Argentina, but speculative length has increased significantly in recent months," said Pawan Kumar, a director of agricultural commodities research at Rabobank in Singapore. "Non-commercials continue to extend their net long position across CBOT soymeal, reaching a 9-month high. We expect the U.S. soybean crop to be larger than (had been) expected and as a result meal futures will retreat."

Wheat is drawing support on concerns over forecasts of rains in the U.S. Plains which could damage the hard red winter wheat crop.

Wheat rose 1.5 percent to $4.73 a bushel, while corn added 0.9 percent to $4.08-1/2 a bushel. Corn earlier reached $4.08-3/4, a level last reached on an most-active contract in late July 2015.

Corn has also been buoyed by brisk export demand and the market will get a fresh demand indication from weekly U.S. export sales figures at 1230 GMT.