Home textile product supplier Menderes Tekstil had threatened and fired employees who showed interest in joining a union, said Framtiden i Våre Hender (The Future in Our Hands), which focuses on ethical and climate issues.
"Health hazardous working conditions have also been reported at the factory," the group said in a statement: in November 2008 a worker had been killed on the premises.
"The conditions are unacceptable, and it is critical that Ikea, which is Menderes' largest customer, now spearheads the efforts to improve working conditions," said Carin Leffler, who heads up the Norwegian branch of the Clean Clothes campaign, which is co-sponsored by Framtiden i Våre Hender.
Contacted by AFP, Ikea said Monday it "takes this matter seriously."
"We are aware of the accidents and complaints, and have over the past year had continuous contacts with union representatives and the supplier," the Swedish company wrote in an email statement.
"Ikea requires that all employees at our suppliers have the right to safe working conditions and should be treated fairly and equally regardless of trade union membership," it wrote.
"Workers' safety is a cornerstone in the Ikea code of conduct," it added.
Neither Ikea nor independent third-party auditors had found "evidence (at Menderes) of serious safety violations or systematic actions towards employees associated to a trade union," the company added.
But it would continue to monitor the supplier closely.
Wal-Mart, along with French retailer Carrefour and British maternity and baby equipment vendor Mothercare also figure among Menderes' customers, said Framtiden i Våre Hender.
They should do more to ensure that the Turkish company improves working conditions, said the group.
"Ikea should, along with the other purchasers, create a so-called 'buyers group' and cooperate with the supplier and the (Turkish) Teksif union to find a quick solution that will secure the workers' fundamental rights," it said.