WASHINGTON, Jan 20 (Reuters) - U.S. home sales tumbled in December as higher prices amid record low inventory continued to shut out some first-time buyers. Existing home sales dropped 4.6% to a seasonally adjusted annual rate of 6.18 million units last month, the National Association of Realtors said on Thursday. Sales fell across all regions. Economists polled by Reuters had forecast sales would fall to a rate of 6.44 million units. Home resales, which account for the bulk of U.S. home sales, declined 7.1% on a year-on-year basis. A total of 6.12 million homes were sold in 2021, the most since 2006. That was up 8.5% from 2020. Sales remained concentrated in the upper price-end of the market. Demand for housing is being fueled by individuals as well as investors who renovate and then resell homes to take advantage of the hot housing market. But rising mortgage rates, supply constraints and higher house prices could make home purchases less affordable this year. There is a record backlog of houses authorized for construction but not yet started, according to government data published on Wednesday. Builders said this week that higher material costs and shortages were adding weeks to typical single-family construction times. The U.S. government in November nearly doubled the duties on imported Canadian softwood lumber to 17.9% from 9% after a review of its anti-dumping and countervailing duty orders. The median existing house price increased 15.8% from a year earlier to $358,000 in December. House prices averaged a record $346,900 in 2021, up 16.9% from the prior year. There were a record low of 910,000 previously owned homes on the market last month, down 18.0% from November and 14.2% from a year ago. At December"s sales pace, it would take 1.8 months to exhaust the current inventory, down from 1.9 months a year ago. A six-to-seven-month supply is viewed as a healthy balance between supply and demand. Properties typically remained on the market for 19 days last month, down from 21 days from a year ago. Seventy-nine percent of homes sold in December were on the market for less than a month. First-time buyers accounted for 30% of sales last month, compared to 31% a year ago.