BEIJING/HONG KONG, July 11 (Reuters) - Onshore bondholders of embattled property developer China Evergrande Group's main unit have rejected a proposal to postpone repayment of a 4.5 billion yuan ($671.04 million) bond, a securities filing made by the unit showed on Monday.

Hengda Real Estate Group Co Ltd, Evergrande's flagship onshore unit, said in a filing to the Shenzhen Stock Exchange it is in talks with bondholders to reach an acceptable agreement as soon as possible.

The 4.5 billion yuan worth of bonds, with a coupon rate of 6.98%, were issued by Hengda via the Shenzhen Stock Exchange in early 2020, and became puttable on Jan. 8 this year, according to previous filings by the company.

Hengda earlier won approval from onshore creditors to extend the puttable date to July 8. The company, however, has now failed to secure approval from those creditors to further extend the payment to Jan. 8, 2023.

Struggling with more than $300 billion in liabilities, Evergrande defaulted on some payments for its offshore bonds last year, but has been trying to extend the timetables on onshore payments to avoid technical defaults.

Last week, Top Shine Global Ltd, an investor in Evergrande unit Fangchebao, said it had filed a winding-up petition against the developer as it had not honoured a pact to repurchase shares from Top Shine in Fangchebao.

The developer has said it would "vigorously" oppose the petition, and that the lawsuit would not impact its offshore debt restructuring plan, expected to land by end of this month. ($1 = 6.7060 Chinese yuan renminbi) (Reporting by Xie Yu in Hong Kong and Beijing newsroom; Editing by Muralikumar Anantharaman)