"Reports indicate that the M23 has wounded civilians, is continuing abductions of children and women, is destroying property and is intimidating journalists and those who have attempted to resist their control," said Eduardo del Buey, the deputy spokesman for the U.N. secretary-general.
A total of 6,000 troops from MONUSCO are in the wider region of North and South Kivu, patrolling areas of particular insecurity, del Buey said.
The United Nations, the African Union and countries in the region are demanding that M23 immediately stop its attacks, del Buey said.
France and Britain also expressed their concerns Tuesday. The British Foreign Office said its minister for Africa, Mark Simmonds, was traveling to Rwanda, DR Congo and Uganda to support efforts to end the crisis.
The eastern part of DR Congo, which includes Goma, has been embroiled in violence since 1994, when Hutu forces crossed the border from Rwanda fearing reprisals following the genocide.
Soldiers from the M23 group were part of the national army as part of peace negotiations brokered in 2009. They broke away from the Congolese army in April, however, complaining of a lack of pay and poor conditions.
Tens of thousands of Congolese, already displaced by previous rounds of fighting in the volatile region, have fled camps around the edges of Goma, according to UNICEF and the medical charity Doctors Without Borders.