WASHINGTON (Reuters) - U.S. mortgage rates jumped to a six-month high this week, suggesting that a recent improvement in home sales could be temporary.

The average rate on the popular 30-year fixed-rate mortgage increased to 6.91%, the highest level since early July, from 6.85% last week, mortgage finance agency Freddie Mac said on Thursday. It averaged 6.62% during the same period a year ago.

"Compared to this time last year, rates are elevated and the market's affordability headwinds persist," said Sam Khater, Freddie Mac's Chief Economist.

Mortgage rates have trended higher despite the Federal Reserve cutting interest rates three times since starting its monetary policy easing cycle in September.

They have risen in tandem with U.S. Treasury yields amid a resilient economy and investor fears that President-elect Donald Trump's proposed policies, including tax cuts, higher tariffs on imported goods and mass deportations, could reignite inflation.

Mortgage rates track the 10-year Treasury note. Sales of previously owned homes surged to an eight-month high in November, mostly reflecting contracts signed in October and possibly September when mortgage rates were mostly lower.

Sales could still rise in December after contracts increased to a 21-month high in November. Increased supply is pulling more buyers into the market, but rising mortgage rates could discourage some homeowners from putting their houses on the market, especially if they would need to buy another home.

Many homeowners have mortgages below 5%. The so-called rate-lock effect could mean fewer homes being listed, reducing inventory and pushing up prices.

This would combine with rising mortgage rates to reduce affordability for many prospective buyers.