Chinese dairy maker Yili said it had started recalling batches of baby formula after authorities found they contained high levels of mercury, in the latest food safety scare to hit the country.

The company began the recall on Wednesday after a national food safety monitoring system detected "abnormal" levels of mercury in the products, state-owned Inner Mongolia Yili Industrial Group said in a statement.

The recall covers baby formula produced from November 2011 to May 2012, according to the statement posted Thursday.

The firm did not state how much baby formula was affected or how mercury -- which is extremely toxic and can harm the brain, heart, kidneys, lungs, and immune system at high exposure -- made its way into the products.

China's quality watchdog said Thursday it had carried out an "urgent monitoring" of 715 samples of baby formula by various producers following the Yili case, but so far no other products were found to be unsafe.

However, authorities were unable to collect samples from 20 firms because they had suspended production of baby formula, the General Administration of Quality Supervision, Inspection and Quarantine (AQSIQ) said in a statement.

It was unclear when they stopped production and whether this was linked to the mercury scare.

Currently, 119 companies in China manufacture baby formula, state media cited Ma Chunliang, an official with AQSIQ, as saying last month.

China's dairy industry is prone to safety scares.

In 2008, milk was at the centre of one of China's biggest food safety scandals when the industrial chemical melamine was found to have been illegally added to dairy products to give the appearance of higher protein content.

Since then, many Chinese people remain suspicious of domestically produced milk after six children died and 300,000 others fell ill in the scandal, which also involved Yili products.

Critics say the hygiene standards that China's dairy farms must adhere to are among the world's lowest, with the levels of bacteria permissible in milk four times as high as in most Western countries.

There have been accusations that the government, keen to ensure China's growing demand for milk is catered to, is giving in to an increasingly powerful dairy industry dominated by Yili and another dairy giant Mengniu.

Yili shares were down by the maximum 10 percent daily limit at 21.85 yuan ($3.4) in Shanghai on midday Thursday, bucking the rising trend in the broader market.