Argentina's central bank is set to hike the country's interest rate as early as this week, a source said, after the economy ministry rolled out a preferential exchange rate for soybean farmers dubbed the "soy dollar" in a bid to promote exports.

The government announced the FX incentive on Sunday to speed up stalled sales of the grain, allowing soy farmers to convert their earnings to local currency at 200 pesos per dollar, far higher than the official rate of 140 pesos.

Argentina is the world's top exporter of processed soy oil and soymeal and the No. 3 exporter of raw soybeans.

A source with direct knowledge of central bank decision making said that the entity would raise rates this week, already hiked sharply in recent months, to tighten liquidity given the expected inflow of funds from the new FX measure.

"The 'soybean dollar' is an exceptional measure that was agreed upon with exporters, the central bank and the Ministry of Economy, but the need to have pesos to buy those dollars makes it necessary to absorb more liquidity urgently," an adviser to the central bank, who asked not to be named, told Reuters.

The central bank declined to comment. Its board normally meets on Thursdays to make policy decisions, though it had been expected to wait until later this month, when August inflation data is due to be released, before increasing the benchmark rate.

"This (latest move on FX) would make the central bank's board not wait for August inflation to be released and move instead ahead of the data, raising the 'Leliq' (bill) rate," the source added.

Reuters reported last week, citing a source and analysts, that the central bank would likely hike the benchmark interest rate to around 75% this month, up from 69.5% now. Inflation is running at over 70% on an annual basis and expected to climb.

Under the new FX measure, Argentina soy farmers will be offered a favorable exchange rate for the month of September, a move intended to spur export sales and bring in much-needed foreign currency.

"The difference in pesos that the central bank must have to buy agricultural dollars will come from somewhere, which suggests that this week we will have a new rise in the reference interest rate," analyst Marcelo Rojas said.