REUTERS - Do you know who helped make your iPhone's battery? Amnesty International says children may have had a hand.

Its report says cobalt used in batteries for phones, laptops and electric vehicles could come from mines in the Democratic Republic of Congo that use child labour. Working with campaign group African Resources Watch, Amnesty accused tech industry giants Apple, Samsung, Sony and others of poor oversight on the supply chain as cobalt goes from mines to smelters and to battery-makers.

The report says, once smelted, the cobalt is exported to China. Then it is sold to battery-makers that claim to supply top-end electronics companies.

In a statement, Apple said it had a zero-tolerance policy towards child labour. It says it is evaluating ways to improve the way it identifies labour and environmental risks.

Samsung said it conducted written evaluations and on-site inspections of all suppliers to certify compliance with human rights, labour, ethics, environment and health standards.

Sony did not respond to e-mailed requests for comment.

The report singled out a smelter in southern Congo owned by Congo Dongfang Mining International. That is a wholly owned subsidiary of China's largest cobalt chemicals producer commonly known as Huayou Cobalt.

Amnesty and Afrewatch said they hoped the research will trigger action.