Jan. 23 (Bloomberg) -- Argentina's peso weakened for a fourth day as the central bank stepped up dollar purchases.

Banco Central de la Republic Argentina bought $2.2 billion in the past three weeks to boost its foreign reserves and to prevent a peso rally that would threaten the country's export growth. The purchases are almost double the $1.2 billion the bank bought in the previous three weeks.

``The central bank has been the main driver in demand'' for the dollar, said Nicolas Coyan, head of trading at Banco Industrial in Buenos Aires.

The peso fell 0.1 percent to 3.097 pesos the dollar at 1:09 p.m. New York time, leaving it down 0.6 percent in the past four days. Coyan predicts the peso will weaken to 3.10 pesos per dollar by March as the central bank keeps buying dollars.

Argentina's foreign reserves climbed to $33 billion yesterday from $18.6 billion in January 2006. Much of the dollars the central bank is buying are flowing in from the country's trade surplus. The government said yesterday that the surplus widened to $1.48 billion in December from $872 million in November.

The surplus was bigger than the $990 million median forecast in a Bloomberg survey of nine economists.

To contact the reporter on this story: Matthew Craze in Buenos Aires at mcraze@bloomberg.net

Last Updated: January 23, 2007 13:11 EST