Since the start of the year, the Argentine peso has weakened 18 percent, ranking it among the world’s worst-performing currencies against the dollar. President Cristina Fernández de Kirchner’s cabinet chief, Jorge Capitanich, insisted on Thursday that the plunge was not a devaluation but the result of the forces of supply and demand.

Still, local news media said the peso closed at 7.75 to the dollar, after weakening earlier on Thursday to about 8.24, with the central bank intervening late in the trading session. Argentina’s international reserves have been tumbling, hitting a seven-year low last week of about $29.5 billion.

Though Mrs. Kirchner has vowed not to devalue the peso, the central bank has let the currency slide gradually in recent months. But there was a change in approach this week, said Gastón Rossi, a former deputy economy minister under Mrs. Kirchner.

“Depreciating in stages was causing the reserves to contract further,” Mr. Rossi said. “The government has said, ‘We’re deliberately not going to sacrifice the reserves anymore.’ ”

With the authorities tightening currency controls in an effort to reduce capital flight, Argentines have resorted to buying dollars illegally, in the black market. The so-called blue dollar rate reached 13 on Thursday, according to the Argentine news media.

With Argentina’s currency weakening, concern is growing that inflation could climb as imported goods become more expensive. The authorities in Argentina say inflation in 2013 was 10.9 percent, while private economists contend that consumer prices actually climbed more than 28 percent last year.

The prospect of abrupt shifts in Argentina’s economy sent tremors into the financial markets of other Latin American countries. In neighboring Brazil, which has the region’s largest economy, the currency weakened more than 1.2 percent to 2.40 reals to the dollar, and the country’s main stock index fell almost 2 percent.