MEXICO CITY, Feb 20 (Reuters) - Mexican conglomerate Alfa on Tuesday posted a fourth-quarter net loss of $652 million, sliding even more from the losses seen a year ago on weak performance from petrochemical subsidiary Alpek.

Alpek sales slipped 31% year-over-year, Alfa said in a news release, as both prices and volumes dropped.

Alpek also was hurt by the "abrupt" December depreciation of the Argentine peso, Alfa said, as the new government of President Javier Milei slashed the currency's value over 50% compared to the dollar as a shock measure to try to put the beleaguered South American nation's economy back on track.

The unit also suffered after the pause of construction on a PTA-PET plant in Corpus Christi, Texas, Alfa said.

Monterrey-based Alfa, whose business lines also include food subsidiary Sigma, said that unit's growth was able to partially offset Alpek's performance.

Sigma revenue rose 10% year-over-year in the fourth quarter on growth in all regions.

Alfa revenue for the last three months of 2023 dropped 13% from the year-ago period to $3.89 billion.

"Going forward, Alfa will be focused on selective asset monetization to reduce debt and complete its transformation," CEO Alvaro Fernandez said in a statement.

Alfa has long said it faces a "conglomerate discount" in which the whole of the firm is valued at less than its parts individually, and it has looked to spin off units in recent years.

Alfa could spin off Alpek as it previously did with its telecommunications unit Axtel and auto parts maker Nemak.

This year, Alpek is set to keep facing a "slow recovery" in the petrochemicals industry, Fernandez added, while Sigma foresees a "stable outlook."

(Reporting by Kylie Madry; Editing by Brendan O'Boyle and Bill Berkrot)