Asda said it saw a 5% drop in the number of shoppers visiting its stores in the first quarter.
The supermarket giant, which has 17 stores in Northern Ireland, said it remained focused on its revival plan, which is seeing it pump £500m into cutting prices for shoppers in a fightback against its rivals and discounters.
It added it was on track with moves to slash costs, having announced in January it would cut hundreds of UK jobs, largely impacting its Leeds head office, which employs 3,000 people.
Andy Clarke, chief executive, pledged to show shoppers a "new Asda face" through a new marketing push, led by a deal with the chef James Martin.
The partnership will see Mr Martin, who recently left the Saturday Kitchen show, provide recipes and cooking advice for families on a budget.
Mr Clarke said: "A new approach to marketing, including our partnership with James Martin, will show customers a new Asda face, which showcases the values on which we have built the business over the years and reinforces our everyday low price value."
Turnaround efforts also include revamping stores and streamlining ranges, he said.
The trading fall marks the seventh quarter of sliding sales.
Asda has been losing ground to rivals amid a price war, with recent supermarket share figures showing it suffered the most out of the big supermarkets.
Falling sales saw Asda lose its position as the UK's second biggest supermarket to rival Sainsbury's last year.