WASHINGTON, Oct 20 (Reuters) - Contracts to purchase previously owned US homes plunged ‌to a record low in ‌September as rising mortgage rates depressed demand for housing.          The ​pending home sales index tumbled 7.3% to 66.0 last month, the lowest level since the series started in 2001, ‌the National Association ⁠of Realtors said on Tuesday. Economists polled by Reuters had ⁠forecast contracts, which become sales after a month or two, slipping 0.5%.                   "Market conditions weakened ​in September ​due to fast-rising ​mortgage rates," said ‌Lawrence Yun, the NAR's chief economist.          Contracts fell in all four regions. Pending home sales dropped 11.3% year-on-year in September.          Mortgage rates have accelerated in recent weeks, with ‌the 30-year fixed rate ​averaging 7.40% last week, ​the highest ​level in nearly three years, ‌data from mortgage finance ​firm Freddie ​Mac showed. The higher borrowing costs, which are tracking Treasury yields as ​the US-Israeli ‌war with Iran fans inflation, are depressing ​home sales.          (Reporting By Lucia Mutikani; Editing ​by Chizu Nomiyama)