Consumers are still holding back on spending, and that uncertainty led to a slowdown in retail sales momentum last week, an index that measures national retail business shows.

The index released Tuesday by the International Council of Shopping Centers and Goldman Sachs has tracked a mix of positive and negative sales results so far in June. For the week ending June 23, sales grew by just 2.7 percent compared to the same period in 2011. The index measures  sales at stores open at least a year and represents about 40 chain stores but does not include restaurant or automobile sales.

 "June has been a tough month for retailers," with mixed results from week to week, Michael Niemira, ICSC's chief economist, said in a statement Tuesday. As the month comes to a close this week, "retailers will continue to deal with consumers' uncertainty in regards to the economy."

There's one bright spot -- falling gasoline prices, Niemira noted. That should help put some extra cash in consumers' pockets and give a boost to retailers, he said.

Still, it won't be enough of a bump to keep the ICSC from lowering its sales forecast for June. The trade group now expects a slightly smaller industrywide gain in store sales than originally projected,  calling for growth of about 3 percent to 3.5 percent compared to June 2011.