By Xie Yu and Liangping Gao

HONG KONG/BEIJING (Reuters) -Chinese regulators' assurances of help in delivering property projects on time failed to convince some homebuyers threatening to stop mortgage payments and investors continued to sell shares in embattled developers on Friday.

A growing nationwide homebuyers' boycott has rekindled investor concerns about the China's slumping property sector, which accounts for a quarter of the economy, and raised fears banks could face hefty writedowns.

Up to 1.5 trillion yuan ($220 billion) of mortgage loans are linked to unfinished Chinese residential projects, ANZ estimated in a report.

Shares in Chinese property developers extended losses, even after the banking watchdog vowed to strengthen coordination with other regulators to "guarantee the delivery of homes" and at least 10 banks said mortgages related to risky projects are relatively small, and risks are controllable.

The regulatory assurances came as homebuyers' threats to withhold payments for stalled property projects have proliferated in official and social media in recent weeks, in a rare show of public discontent.

The Hang Seng Mainland Properties Index tumbled 5% on Friday, dragging the Hong Kong benchmark index down 2.2%.

Among those hardest hit, shares in Shanghai-based CIFI Holdings plunged 14.3%, while top developer Country Garden Holdings Co Ltd fell 8.6%.

Developers' bonds also took a heavy hit.

A 2026 dollar bond of Yuzhou Properties traded at 6.354 cents on the dollar on Friday afternoon, down from 6.861 a day ago, while a 2024 bond of Xinyuan Real Estate dropped to 11.125 from 12.425.

Onshore, a yuan bond of Powerlong Real Estate and Sino-Ocean Group < 3377.HK> slid 20% and 16%, respectively.

Government assurances were not enough to convince at least some homebuyers threatening to stop mortgage payments.

One homebuyer in the east-central Chinese city of Zhengzhou said on Friday that while local authorities assured buyers that the developers would resume construction soon, there hadn't seen any action on the ground.

"We don't have any other way to voice at the moment and we're still in a desperate situation," said the person, who declined to be identified due to sensitivity of the matter.

In a letter issued to the Xinyuan homebuyers in Yingyang, Zhengzhou, on Friday and seen by Reuters, the housing regulator said it along with other government departments had reached an agreement with the developer and the contractor that some funds would be paid to resume construction.