UPDATE 1-Walmart optimistic for "challenging" US holiday quarter

Sept 5 (Reuters) - Wal-Mart Stores Inc is optimistic about its prospects in the United States for the important holiday season even though it expects the period to be a challenging one, Walmart U.S. Chief Executive Bill Simon told investors on Wednesday.

Walmart U.S., the largest unit of the world's largest retailer, faces increased pressure from a wide variety of rivals. As chains such as Dollar General Corp emphasize $1 items, grocers including Supervalu Inc have cut some of their everyday prices, and competitors such as Target Corp are stocking exclusive items to entice shoppers.

"We're optimistic about the fourth quarter, we do expect it to be challenging," Simon said at a Goldman Sachs retail conference. "The retail environment is getting very aggressive."

Simon's comments, which came after four consecutive quarters of sales gains at existing Walmart U.S. stores, were also broadcast over the Internet.

One way Walmart is trying to boost its growth is through its website, which offers free shipping with certain minimum purchases and waives shipping fees for shoppers who pick up their items in a Walmart store.

Walmart's ecommerce business grew at or above ecommerce rates in the United States during the first half of the year, taking market share from competitors, Simon said.