Wal-Mart Stores Inc appears to have hit a speed bump in the race to grow online.

The retailer's e-commerce sales rose 8 percent in the fiscal fourth quarter ended Jan 31, for a fourth straight period of decelerating growth.

Growth has gone from 17 percent in the first quarter, to 16 percent in the second and 10 percent in the third. The numbers exclude the impact of currency swings.

Wal-Mart, which reported quarterly results on Thursday, said the slowdown was due to weakness in China, the United Kingdom and Brazil. It does not break out figures for individual markets, including the United States.

The numbers suggest Wal-Mart is slipping further behind online leader Amazon.com Inc, whose North America sales grew 24 percent and international sales grew 22 percent in the fourth quarter on constant currency terms.

"It's definitely a challenging trend," said Keith Anderson, vice-president at e-commerce analytics firm Profitero Inc. "The really troubling aspect of it is the fourth quarter. It's the holiday period."

Amazon outstripped industry-wide US online growth in the fourth quarter, which was 14.7 percent, according to Department of Commerce data. Amazon now has a 23.7 percent share of the US online market, compared with Wal-Mart's 2.5 percent, according to retail consultancy Conlumino.

Asked about bridging the gap with Amazon, Wal-Mart e-commerce head Neil Ashe said he was focused on "building the customer relationship" through the retailer's mobile app, services like grocery pickup, and by growing assortment.

"The customer is reacting positively to that," he said.

The soft showing in Brazil reflects an economic slump impacting all retailers, while Wal-Mart's struggles in China are partly due to gains by Alibaba Group Holding Ltd, a factor also likely cutting into Amazon's sales there, Profitero's Anderson said.

But Wal-Mart's slowing growth also highlights broader challenges of logistics and price competitiveness, including in the key US market, Anderson said.

According to a recent Profitero survey of 2,461 products, Amazon had lower online prices than both Target Corp and Wal-Mart across six product categories.

Anderson said Amazon also has the upper hand on selection and delivery times.

Wal-Mart contests that it is more expensive, saying its own survey of a wider assortment of items shows it has the same or lower price than leading online competitors four out of five times.

Wal-Mart has spent aggressively to become more efficient, earmarking $2 billion over two years to build out its e-commerce infrastructure, including on dedicated online fulfillment centers that can sort and ship packages at a lower cost.

Keeping pace with Amazon will not be easy. The Seattle-based retailer spent $4.6 billion on capital expenditure last year, and continues to expand its Prime program, which is estimated by some analysts to have 50 million members worldwide.