The UN Secretary General's Special Representative for the Democratic Republic of Congo (DRC) Bintou Keita warned of the "very real risk" of the conflict in eastern Congo spreading.
She pointed to increasing territorial gains by the M23 militia in the eastern Congolese province of North Kivu, and noted Rwanda has also increased its support for the militia, in her report to the UN Security Council on Monday.
In the subsequent debate, the US ambassador to the UN referred to credible reports of more than 4,000 Rwandan soldiers in Congolese territory who have been involved in attacks.
The resource-rich eastern Congo and the province of North Kivu in particular have been the scene of armed conflicts for 30 years.
Rwanda refutes the repeated accusations of military intervention in the conflict and, conversely, accuses Congo of failing to adequately protect the Tutsi ethnic group in the east of the country.
"We are witnessing in the DRC one of the most severe, complex and neglected humanitarian crises of our times," Keita said of the 6.9 million internally displaced in North Kivu alone.
Gender-based violence continues to rise and last year, almost 123,000 cases were reported, an increase of 3%, she said, adding this is probably just the "tip of the iceberg" as many victims stay silent out of shame.
The significant increase in sexual violence against children is alarming, said Keita.
Meanwhile, a two-week humanitarian ceasefire in North Kivu, which came into force at the weekend, appears to be largely holding, according to Congolese media reports.
However, security sources said the M23 militia was reinforcing its positions, according to Radio Okapi. Humanitarian volunteers often have no access to displaced people.
The United Nations Organization Stabilization Mission in the Democratic Republic of the Congo (MONUSCO) is due to leave the country at the end of the year.
Before UN soldiers withdraw, the security situation on the ground must be taken into account in order to ensure a "responsible" withdrawal, the Congolese Foreign Ministry said, while the UN special representative also warned of a security vacuum in the region.
The Congolese army is facing increasing problems within its own ranks in view of the M23 successes. In recent weeks, military courts have sentenced more than 50 soldiers to death for desertion or "cowardice before the enemy."
Only on Monday, 22 members of the army were sentenced to death in Lubero in North Kivu, according to the Actualité newspaper.