Mortgage rates in the US increased for the first time this month.

The average for a 30-year, fixed loan was 6.39%, up from 6.35% last week, Freddie Mac said in a statement Thursday.

While buyer demand remains strong in certain US cities, high borrowing costs and a general lack of homes for sale are limiting purchases in many markets. Transactions for previously owned homes fell to a three-month low in April with inventory tight.

“Economic crosscurrents have kept rates within a 10 basis-point range over the last several weeks,” Sam Khater, Freddie Mac’s chief economist, said in the statement. “While affordability remains a hurdle, homebuyers are getting used to current rates and continue to pursue homeownership.”

Talks over the US debt ceiling have been weighing on financial markets recently as lawmakers race to avoid a default that could come as soon June 1 if a deal isn’t reached.

“Though a default remains unlikely, the closer we get to a possible event date without an increase in the debt limit, the more likely households will be affected by higher interest rates,” said Hannah Jones, a Realtor.com economic data analyst.