Japanese clothing giant Uniqlo came under fire Tuesday for buying supplies from Chinese factories accused of putting workers at risk in unsafe conditions, with sewage on the factory floor, extremely high temperatures and poor ventilation.

The claims made by a Hong Kong-based human rights group come as the apparel chain pursues aggressive expansion plans in a bid to challenge international brands like Zara, H&M and Gap.

Uniqlo, a unit of Fast Retailing Co, said that while it had "different views on some of the issues" in the report, it had launched an investigation and acknowledged the probe by Students and Scholars Against Corporate Misbehaviour (SACOM) "found several problems."

SACOM accused the clothing firm, known for its cheap chic clothing basics, of buying from two suppliers in South China's Guangdong Province that made employees work long hours for low pay in unsafe conditions.

The group, which carried out its investigation between July and November 2014, said the Uniqlo suppliers also had a tough management style and did not allow workers to voice their concerns.

"The factories have neglected work safety, putting workers at risk," it said.

The rights group in particular cited "extremely high shop floor temperatures, dirty sewage flowing all over the floor, unsafe facilities, poor ventilation with dense cotton dust filling the air, irritating smells and high risk of electricity leakage."

The group noted that "topless workers put the heavy pigments into the hot dyeing tent without wearing any protective gear" and said that its investigators "saw a number of workers falling down from the chairs while handling the knitting machines."

"Uniqlo as the key buyer of these two factories has violated its commitment to corporate social responsibility," SACOM said in a statement.

The group said one employee worked up to 14 hours per day, ironing between 600 and 700 shirts, for wages of 0.29 yuan ($0.05) per shirt.

SACOM has in the past carried out investigations into working conditions for employees of Taiwan-headquartered technology giant Foxconn, London Olympic Games suppliers, and factories that sell to Disney.

Fast Retailing said in a statement that it had opened an investigation into the allegations.