KINSHASA (Reuters) - Democratic Republic of Congo has summoned the Angolan ambassador over the expulsion of thousands of Congolese migrants from Angola during a crackdown on artisanal diamond mining this month, Congo’s foreign minister said on Wednesday.

Congolese migrants and officials have said dozens of people were killed during an operation by Angolan security forces and an allied ethnic group this month. Angola denies this and says many Congolese migrants left voluntarily.

“The Government of the Republic of Congo urges...Angola to conduct a comprehensive investigation to establish who is responsible for these wrongful acts,” Foreign Minister Leonard She Okitundu said in a statement.

Congo’s ambassador to Angola has been summoned on Wednesday by Angolan authorities to be briefed on the case, he said.

The dispute comes at a time of worsening relations between the government of Congolese President Joseph Kabila and its erstwhile ally to the south.

Two decades ago, Angola stepped in to rescue Kabila’s father, then president Laurent Kabila, from a Rwandan-backed rebel assault.

But Angola has recently become frustrated by Kabila junior’s handling of several crises, including instability caused by his failure to step aside when his mandate ended in late 2016, and a conflict in Congo’s Kasai province that sent thousands of refugees over the border.

Congo government spokesman Lambert Mende said about 28,000 Congolese had been “brutally” expelled in violation of an agreement between the two countries on freedom of movement.

Angolan Police Commissioner Antonio Bernardo dismissed the accusations of violence on Wednesday, and said that the border agreement only allowed for visits up to 48 hours.

“Angola is a sovereign country,” he told Reuters. “The defense of sovereignty necessarily includes the safeguarding of national security, the protection of its natural resources.”

He said the Congolese migrants were residing in the country illegally, and had already received “selfless assistance” despite their status, and had left of their own free will.

Angola has in recent weeks stepped up efforts to tackle diamond smuggling and raise more revenue from the lucrative sector.