NANJING -?China's maritime authorities have confined a foreign cargo ship as they investigate a chemical spill that partially contaminated the drinking water source of a city downstream the Yangtze River in East China's Jiangsu province, officials said Thursday.

The government of Zhenjiang city on Tuesday confirmed that on February 3 phenol levels collected in water samples were excessive, and the pollution caused tap water in some parts of the city to emit a pungent smell, sending panicked residents to stockpile bottled water from supermarkets.

The authorities "highly suspect" that a South Korea cargo ship that had docked at Zhenjiang spilled the chemical when a valve was not properly closed, officials said. The departments of maritime affairs, environmental protection, and border quarantine are involved in the probe.

Xinhua reporters on Thursday were denied access to the suspected cargo, now docked close to the city of Nantong. Local maritime officials declined to comment on the case.

The governments of Nantong and Zhenjiang both said water samples of the two cities on February 4 did not detect excessive phenol. Meanwhile, the authorities have ensured residents that the supply of purified bottled water is sufficient.

Phenol, also known as carbolic acid, is an organic compound that can irritate eyes and skin. Soluble in water, if absorbed in large amounts it can damage the liver and kidneys.