Adds details on quarterly results in paragraphs 2-5

April 24 (Reuters) - Valero Energy VLO.N reported a first-quarter loss compared with a year-ago profit on Thursday, weighed down by lower refining margins during a quarter marked by elevated maintenance and turnaround activity across the U.S. refining sector.

U.S. refineries typically undergo seasonal maintenance, known as turnaround activity, in the first quarter to prepare units for the higher output needed during the summer driving season.

This maintenance-related downtime temporarily reduces refinery utilization, denting their ability to capture revenue.

The company said its quarterly refining margins fell to $2.49 billion from last year's $3.53 billion.

Valero also recorded $1.1 billion in pretax impairment charges related to its West Coast assets.

Valero, which kicked off refiners' earnings, said net loss attributable to stockholders was $595 million, or $1.90 per share, in the three months ended March 31, compared with last year's profit of $1.2 billion, or $3.75 per share.

(Reporting by Arunima Kumar in Bengaluru; Editing by Maju Samuel)

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