The Democratic Republic of Congo is losing a fifth of all of the mining revenues because of corruption and mismanagement, a campaign group has said.

In a report published on Friday, Global Witness said at least $750m paid by companies to the Congo's tax agencies and state-owned mining company Gecamines disappeared between 2013 and 2015.

At least some of the funds were distributed among corrupt networks linked to President Joseph Kabila, the group said.

The losses deprived the state of funds that should have been used on public services, it added.

Congo is Africa's top copper producer and the world's biggest supplier of cobalt, which is used in mobile phones and electric cars. It also produces coltan, diamonds, tin and gold.

However, it remains one of the poorest countries in the world.

"Congo's mining revenues should be helping to lift its people out of poverty, but instead huge sums are being siphoned away from the public purse and into unaccountable agencies headed up by people with ties to political elites," said Pete Jones, a senior campaigner at Global Witness.

The findings come amid renewed political unrest in the Congo after the country's election commission announced that a vote to replace Kabila was unlikely to take place this year.

The political turmoil and persistently low prices for Congo's exports has caused the franc to lose 40 percent of its value in the past year.

Global Witness said a key cause of discontent with the regime was the chronic lack of funding by the government in basic services such as schools, hospitals and roads.

The main culprit in the diversion of funds was Gecamines, a state-owned company tasked with the exploration, research and production of Congo's mineral deposits, its report said.

Gecamines is "haemorrhaging money in suspect transactions ... while simultaneously failing to make any substantial contribution to the national treasury or invest in its own mining operations," the report said.

Congo's "secretive" tax agencies, meanwhile, "keep back a portion of mining revenues for their 'own funds' rather than transfer it to the treasury," it added.

The missing revenues rise to $1.3bn when other state bodies and a now-defunct provincial tax body were included, it said.

Lambert Mende, a spokesman for Kabila, said the judiciary would deal with corruption cases.

"We have institutions in charge of handling such cases. The courts are working and if it's true that money from mining revenues is being embezzled, those involved will be brought to court and be prosecuted," Mende told the DPA news agency.