Rebels occupying Twangiza Mining's gold concession in eastern Democratic Republic of Congo (DRC) have looted at least 500 kg of gold since May, the company told Reuters, accusing some of its own employees of aiding the theft.

At current prices, the looted gold is worth about $70 million.

The mine is located in South Kivu province, where Rwandan-backed M23 rebels launched a lightning offensive this year that allowed them to seize more territory than ever before. They took control of the mine in May.

"With the help of some employees, they transported the first batch of more than 50 kg of gold in a very short time," Twangiza Mining said in a written response to Reuters' questions on Monday about losses incurred since the M23 takeover of the mine.

"Since the occupation, they have obtained at least 500 kg of gold and transported it secretly through underground routes," the company said.

The M23 did not immediately respond to a request for comment.

Congo-based Twangiza Mining, which describes itself as a Chinese company, said it has lost more than 100 kg of gold per month since the takeover, in addition to $5 million worth of equipment and materials.

The company is preparing to file a formal complaint with an international arbitration tribunal and Congolese authorities, and has declared a case of force majeure, it said.

She accused the rebels of evicting residents, demolishing churches and using Rwandan technicians to extract geological data to resume and expand mining.

"There are more than 150 workers left on site. We cannot contact them," the company said.

The Rwandan government did not immediately respond to a request for comment.

A drone strike on October 15 destroyed the mine's power generation infrastructure. It is unclear who was responsible for the strike.

Fighting in eastern Congo has killed thousands and displaced hundreds of thousands this year.

Armed groups have seized several mining sites in mineral-rich eastern Congo, according to UN investigators.

According to a UN Security Council report released last year, M23 rebels were earning about $300,000 a month from mineral taxes levied in the coltan-rich Rubaya region.

US President Donald Trump brokered a peace deal between Congo and Rwanda in June as part of an effort to stabilize eastern Congo and attract Western mining investment.

Rwanda has consistently denied supporting the M23 rebels, despite repeated allegations by UN experts and regional governments.

Qatar hosted direct talks between Congo and the M23.

Both sides missed the August deadline for reaching a peace agreement under the process, but on October 14 they agreed on a monitoring mechanism for a possible ceasefire.