UN chief Antonio Guterres called Sunday on Rwandan forces to withdraw from the Democratic Republic of Congo and halt support for fighters advancing on the key Congolese city of Goma.
M23 fighters backed by several thousand Rwandan troops have been quickly advancing toward the city, which lies along DRC's eastern border and is home to more than a million people.
Several foreign peacekeepers have been killed in the mounting violence around Goma.
"The Secretary-General is deeply concerned by the escalating violence" and "calls on the Rwanda Defence Forces to cease support to the M23 and withdraw from DRC territory," said a statement from his spokesman Stephane Dujarric.
Guterres had previously referred to a UN experts' report citing Kigali's backing of the M23, but had not explicitly called on Rwanda to withdraw from DRC territory.
In his statement Sunday, made after three UN peacekeepers in eastern DRC had been killed within 48 hours, Guterres emphasized that "attacks against United Nations personnel may constitute a war crime."
The UN meantime has begun to evacuate "non-essential" staff from the major city of Goma in eastern DRC, a United Nations source told AFP.
During an emergency meeting of the UN Security Council session on Sunday, permanent member states France, Britain and the United States called on Rwanda to pull its forces back.
But others, including China and the African nations holding rotating council seats, did not specifically name Kigali.
The Security Council as a whole has yet to accuse Rwanda of taking part directly in the conflict, simply underlining the importance of the DRC's territorial integrity.
But French ambassador to the UN Nicolas de Riviere indicated Sunday he was working on a Security Council statement that would "call a cat a cat."
He urged the Council to condemn what he said was a grave threat to regional peace and security.
Congolese foreign minister Therese Kayikwamba Wagner went further, urging the Council to impose sweeping economic and political sanctions on Kigali.
She accused Rwanda of having sent new troops into eastern DRC on Sunday, actions which she said amounted to a "declaration of war."
But Rwanda's ambassador to the UN, Ernest Rwamucyo, rejected the accusations, accusing Kinshasa of being responsible for the deteriorating situation and failing to make a "genuine commitment to peace."
He suggested that the UN peacekeepers in the DRC had joined a "coalition" seeking regime change in Rwanda.
The fighting in the region has forced some 230,000 to flee their homes.
Eastern DRC has vast mining resources and is a complex landscape of rival armed militias which has seen violence ebb and flow since the 1990s.