Supermarket sales volumes rose for fifth straight month as promotional spending increased to 34% of sales and price deflation scraped along at an all-time low, the latest Nielsen market share data showed.Sainsbury's suffered the smallest decline in year-on-year sales to stand out at the best performer among the Big Four supermarkets in the 12 weeks ending 25 April, echoing reports from rival data-crunchers at Kantar, with a 0.1% fall.The worst result was for Asda, which saw a 2.5% decline in year-on-year sales over the 12-week period.Total grocery sales volumes swelled 0.4% during the four weeks ending 25 April 2015 versus the same period a year ago.As general inflation, sparked by lower oil prices, combined with the raging supermarket price war, food prices fell for the fourth consecutive month, with food deflation remaining at an all-time low of -0.9%.But, as a result, sales value during the four-week period fell 1.2% versus the same period a year ago."Food deflation and falling spend-per-visit are the biggest challenges facing UK supermarkets," says Mike Watkins, Nielsen's UK head of retailer and business insight."Although retailer price wars are contributing to sustainable growth in the amount people buy, price is still less of a differentiator for building loyalty to a supermarket, than the overall store and shopper experience."He pointed out that promotions have increased to 34% of sales in order to drive incremental spend - good news for the advertising industry - as big grocers fight to drive incremental spend.But he suggested retailers will need to work much harder at communicating why shoppers should choose them, beyond just price."Lidl, for example - currently the biggest spender on advertising of the supermarkets - is now focusing more on its quality and fresh food messaging than simply low prices."Spending even more than Tesco's £4.5m, the German discounter, which grew sales 7.1% in the 12-week period, was the biggest spender in April on TV and press advertising for a second consecutive month, spending £5.2m.With Aldi's sales growing 19.6% year-on-year, the two Continental discounters have a combined market share of 11% now, well over double the 4.9% from a year ago, suggesting there is still plenty of fight in the industry's price war.Marks & Spencer, up 3.6%, Waitrose, up 1.7%, and Iceland, up 0.9%, were the only other supermarkets to see a year-on-year increase in sales.Watkins concluded: "If deflation continues, we anticipate volumes increasing over the summer as people channel savings made into affordable indulgences from the supermarket, not just on food consumed out of the home."