* Noble shares close 8 percent lower on weak earnings

* posted on unexpected quarterly loss due to the write-downs

* Investors concerned about complexity of commodity firms (Updates to change headline)

By Anshuman Daga and Umesh Desai

SINGAPORE/HONG KONG, Feb 27 (Reuters) — An unexpected $440 million asset write-down by Noble Group Ltd, following allegations of improper accounting by an obscure research firm, has raised investor concerns about how much leeway commodity traders have in valuing their assets.

Singapore-listed Noble, one of Asia’s biggest commodity traders, lost $1.1 billion of its market value this month after Iceberg Research alleged it had overstated the value of commodities it held by at least $3.8 billion. Noble has rejected the charges.

The company then reported an unexpected quarterly loss on Thursday due to the write-downs, sending its shares tumbling to a six-week low. The shares have fallen a total 19 percent since Iceberg released its first report attacking Noble on Feb. 13.

Analysts said that while the timing of the write-down was unfortunate, the practice was not unusual for commodity traders which have to assess the value of their holdings based on the volatile prices of products such as coal and iron ore. The criteria these companies use, however, are often subjective.

Commodity companies add a layer of complexity because of the accounting standards requiring them to re-value the biological assets, said Roger Tan, chief executive of Voyage Research.

The bigger issue is the complexity of their company structures and the amount of information and the way they present the information to the market.

The Hong Kong based firm took total impairments, write-offs and provisions of $438 million on a range of assets and investments in 2014, following its annual year-end review process.

Noble CEO Yusuf Alireza said almost half the write-down was related to investments in Australia’s Yancoal Coal, and reiterated that it was not connected to the Iceberg report. Noble said a disgruntled employee was behind Iceberg.

The market is definitely a little bit more comforted that they adjusted their numbers and took an impairment, said Carey Wong, an analyst at OCBC.

While a lot of people don’t really put much into Iceberg’s allegations, what was equally important was the fact that they brought to light the weak pricing of some of Noble’s associates.

Noble is the second major Singapore-listed commodity trader to have its accounting practices challenged by a small research firm. Olam International was hit by a critical report from Muddy Waters in late 2012, which prompted state investor Temasek to take majority control.

Noble’s bonds were initially sold off before recovering on short-covering. Bonds due 2020 fell to a low of 101.5 today from Thursday’s high debt level of 105, before recovering to 102.5/103.

Credit investors don’t worry about book values or impairments. We care about cash flows, leverage and debt maturities, said Dhiraj Bajaj, Hong Kong-based portfolio manager at Lombard Odier.