WASHINGTON, Jan 15 (Reuters) - U.S. mortgage rates fell significantly this week after the Trump administration initiated purchases of mortgage-backed securities to boost housing affordability.

The average rate on the popular 30-year fixed-rate mortgage declined to 6.06%, the lowest level since September 2022, from 6.16% last week, mortgage finance agency Freddie Mac said on Thursday. It averaged 7.04% during the same period a year ago.

President Donald Trump last week ordered the Federal Housing Finance Agency - which oversees Freddie Mac and another mortgage finance giant Fannie Mae - to purchase $200 billion of bonds issued by the two companies.

FHFA Director William Pulte said last week they had started with a $3 billion initial round of purchases. Trump is under pressure to bring down costs, including for housing, as he and his fellow Republicans face a tough battle to retain control of the U.S. Congress in this year's mid-term elections.

Trump has also proposed banning institutional investors from buying single-family homes. The average rate on the 15-year fixed-rate mortgage fell to 5.38% from 5.46% in the prior week. It averaged 6.27% during the same period a year ago.

(Reporting By Lucia Mutikani; Editing by Hugh Lawson)

((Lucia.Mutikani@thomsonreuters.com; 1 202 898 8315; Reuters Messaging: lucia.mutikani.thomsonreuters.com@reuters.net/))