HONG KONG (Reuters) - Developer Kaisa Group Holdings missed a deadline to pay a US$26 million bond coupon, fuelling investors' concerns about the creditworthiness of Chinese property firms amid a downturn in the market.

The payment was due on midnight Hong Kong time on Thursday, but traders said Kaisa had a 30-day grace period to resolve the situation.

Failure to pay the coupon on Kaisa's 2020 bond could trigger the first dollar bond default by a Chinese property issuer and Kaisa's troubles have sent other China property bonds tumbling.

Chinese offshore property debt outstanding amounts to US$55.7 billion, according to Dealogic, and accounts for roughly half the high-yield borrowers in Asia outside of Japan.

The property bonds had been one of the world's hottest investments because of the high yields on offer and a belief that any Chinese borrower running into trouble would be kept afloat by bailouts or debt extensions.

"The main thing now is payment before the grace period. Lateness does not matter as the market has already priced that in the bonds," said a Hong Kong-based trader who declined to be named as he was not authorised to speak to the media.

Shenzhen-based Kaisa has declined to comment on its debt issues. The company, one of China's smaller listed developers, warned last week it may default on more debt after it failed to repay HSBC a HK$400 million loan that was due on Dec. 31.

Kaisa's stock remains suspended in Hong Kong but its bonds have been plummeting.

Trading in the 2020 bond remained subdued on Friday at around 30 cents on the dollar. All Kaisa's outstanding bonds have lost two-thirds of their value over the past month and its shares have been suspended since the end of December, after its was hit by the resignation of senior executives and a government-imposed block on some of its projects.

China's property market, a key driver of economic growth with also affects more than 40 other sectors from cement to furniture, is grappling with oversupply and tighter credit.

Investment in the sector grew at its slowest pace in over five years between January to November, data showed last month.

The government is also increasingly scrutinising developers in the southern Guangdong area as part of a widespread campaign against corruption, heightening investors' concerns.

On Friday, newly listed developer Logan Property said the authorities had since 2010 locked three units, now worth some 6 million yuan at a residential project in the southern city of Shenzhen in Guangdong. It said the lock was due to land problems at a nearby gas station and that the issue would not have any impact on its business, but traders said the news was likely to fuel market jitters about developers in Guangdong.

Logan Property bonds due 2017 are indicated at 75/80 cents on the dollar, down from their year-end level of 98 cents.