More than 20% of the Democratic Republic of Congo's mining revenue is being lost due to corruption and mismanagement, a campaign group says.

According to a Global Witness report, the money is being distributed through corrupt networks linked to President Joseph Kabila.

At least $750m (£580m) has gone missing over the past three years, it says.

The government has not commented but has previously denied allegations of corruption in its mining sector.

DR Congo is Africa's biggest producer of copper and the world's largest supplier of cobalt used in batteries for electric cars.

It is also rich in gold, diamonds and coltan, used in mobile phones, but its people remain among the poorest in the world following years of conflict and mismanagement.

"Congo's mining revenues should be helping to lift its people out of poverty," says Pete Jones, a Global Witness senior campaigner.

According to the report, much of DR Congo's mining revenue goes missing after being paid to the state-owned mining company, Gécamines.

The head of Gécamines, a close ally of President Kabila, has denied allegations of corruption and insisted the company is transparent.

Other alleged culprits identified by the report are the country's tax agencies which are legally allowed to hold back a percentage of the taxes they impose.

This creates avenues for corruption through inflating how much of the fines they can keep for themselves, Global Witness says.

The report says that if the money not reaching the state coffers through corruption, mismanagement and an ineffective tax system was collated, from 2013 to 2015, then the amount rises to $1.3bn.