NEW YORK (Reuters) - Weekly U.S. mortgage refinancing activity fell last week to its lowest since June as home borrowing costs rose to their most expensive level in about three months, the Mortgage Bankers Association said on Wednesday.

The trade group's seasonally adjusted index on refinancing activity fell 8 percent in the week ended Sept. 16 to its lowest level since the week of June 24.

The average contract interest rate on 30-year fixed-rate mortgages with conforming loan balances of $417,000 or less climbed to 3.70 percent, which was its highest since 3.75 percent in the week ended June 24, the Washington-based group said.

Meanwhile, MBA's seasonal adjusted index on applications for loans to buy a home, which is seen as a gauge on future housing activity, fell 7 percent from a week earlier.

This week's domestic housing data were mixed.

The government said on Tuesday housing starts fell more than expected by 5.8 percent to an annualized 1.14 million units in August. Analysts blamed the drop on the flooding in Texas and Louisiana, which cut into construction in the South.

On the other hand, the National Association of Home Builders and Wells Fargo said on Monday their index on their members' confidence increased to 65 points in September, matching its strongest level since October 2015. This compared with a downwardly revised 59 in August.

The MBA's index on overall mortgage activity fell 7.3 percent on a seasonally adjusted basis from a week earlier.

The refinance share of mortgage activity edged up to 63.1 percent last week, compared with 62.9 percent the previous week.

(Reporting by Richard Leong; Editing by Chizu Nomiyama)