Argentina's President Javier Milei vowed on Sunday to "accelerate" his libertarian reforms after a crushing defeat in Buenos Aires provincial elections, ahead of highly anticipated midterms.

The 54-year-old economist has slashed public spending, dismissed tens of thousands of public employees and led a major deregulation drive since taking office in December 2023.

He acknowledged his party's "clear defeat" by the centre-left Peronist movement in the elections to the legislature of Buenos Aires province, the country's economic powerhouse.

Milei admitted to unspecified "mistakes" which he vowed to "correct" but said he would not be swayed "one millimetre" from his reform agenda.

"We will deepen and accelerate it," he said at a muted election night event in the resort of Mar del Plata, where he took the stage in silence, in marked contrast to his usual dramatic entrance to rock music.

With 91 percent of the votes counted, the centre-left Fuerza Patria coalition had taken over 47 percent of the vote against nearly 34 percent for Milei's ruling La Libertad Avanza (LLA), official results showed.

Buenos Aires's votes are telling as a bellwether for Argentina.

The province contributes more than 30 percent of Argentina's GDP and accounts for 40 percent of all eligible voters.

The 13-point gap between Milei's party and the left was far greater than opinion polls had predicted.

Turnout in the election was high, at around 63 percent.

The result poses major concerns for Milei, coming just six weeks before midterm elections.

Some members of his party downplayed the extent of the defeat, pointing out that the LLA had nonetheless increased its share of legislators in Buenos Aires. (AFP)