Democratic Republic of Congo's government on Monday said 272 civilians were killed in a massacre in the eastern town of Kishishe last week, raising the death toll from a previous estimate of 50.
The government blamed the killings on the M23 rebel group, which has denied responsibility. It also said the rebels were backed by members of the Rwandan army, a frequent accusation by the Congolese government which Rwanda has consistently denied.
Rwandan authorities could not be reached for comment.
Congo's army and the M23, a Tutsi-led militia, have been locked in fighting for months in the country's east.
The alleged massacre occurred on Nov 29 in Kishishe, in North Kivu province. The death toll was announced by Congolese industry minister Julien Paluku, speaking at a press briefing with government spokesman Patrick Muyaya.
"I cannot give details of the attack. An investigation has been opened by the attorney general and we are awaiting the results of the investigators," said Muyaya.
"What we do know is that children were killed in an Adventist church and a hospital," he said.
The United Nations said last week it had received reports of a high number of civilian casualties during clashes between the M23 and local militias in Kishishe, but did not give figures.
In its own account of the events, the M23 said that 21 fighters were killed from an enemy coalition, and that eight civilians were killed by stray bullets.
A group of United Nations experts said this year it had "solid evidence" that Rwandan troops were fighting alongside the M23 and providing it with weapons and support, which Rwanda denied.
The leaders of Congo and Rwanda have met several times to attempt to solve the crisis, including recently in Luanda where they agreed on a ceasefire. But fighting has continued since.
US Secretary of State Antony Blinken said on Monday he had had a "productive conversation" with Rwandan President Paul Kagame about the need for peace and security in eastern DRC.
"The United States urges Rwanda to honour commitments made in Luanda, including ending Rwanda's support to M23," Blinken said on Twitter.
But Rwanda's foreign minister Vincent Biruta said that "differences in understanding of the issue remain".
"M23 should not be equated to Rwanda. It is not Rwanda's problem to solve," he said on Twitter.