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

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

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

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

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

M23 did not immediately respond to a request for comment.

Twangiza Mining, which is headquartered in Congo and describes itself as a Chinese firm, said it lost over 100 kg of gold a month since the takeover, in addition to $5 million worth of equipment and materials.

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

It accused the rebels of expelling 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 get in touch with them," the company said.

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

A drone strike on October 15 destroyed power generation infrastructure at the mine. It is not clear who was responsible for the drone strike.

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

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

A U.N. Security Council briefing last year said M23 rebels were earning around $300,000 monthly from mineral taxes in the coltan-rich Rubaya region.

U.S. President Donald Trump brokered a peace deal between Congo and Rwanda in June as part of an effort to stabilise eastern Congo and bring in Western mining investments.

Rwanda has consistently denied backing M23 rebels, despite repeated allegations from U.N. experts and regional governments.

Qatar has been hosting direct talks between Congo and M23.

The two sides missed an August deadline for a peace deal as part of that process but on October 14, they agreed to a monitoring mechanism for an eventual ceasefire.