(Reuters) - Amazon.com Inc said it would hire more than 120,000 seasonal workers in the United States for the holiday season, 20 percent more than last year, highlighting the growing threat the e-commerce giant poses to traditional retailers.

U.S. retailers such as Macy’s Inc, Target Corp and Kohl’s Corp have said they plan to hire fewer temporary workers or to keep seasonal employment levels little changed this holiday season.

More than 14,000 seasonal positions were transitioned to regular, full-time roles after the holidays last year, and the company expects to increase that number this year, Amazon said on Thursday.

The U.S. National Retail Federation earlier this month forecast a 3.6 percent rise in holiday sales this year, with online sales expected to climb 7 percent to 10 percent.

U.S. brick-and-mortar retailers’ biggest challenge in recent years has been tackling the growth of online retailers, specially Amazon, which offer the same products at lower prices and have made shopping more convenient.

They are also keeping sales expectations and inventories low - and hiring light - ahead of the holiday season to avoid a repeat of last year, when unusually warm weather hit sales and piled up unsold goods.