Witnesses said U.N. troops charged with defending Goma residents watched Tuesday as M23 rebels walked through the streets of the city, home to many humanitarian aid agencies operating in the region, The Wall Street Journal reported.
Reports indicated that Congolese fighters fled as the rebels, reportedly supported by Rwanda and Uganda, advanced.
Also on Tuesday, the U.S. State Department urged the leaders to "engage in a direct and honest dialogue" to find a political resolution.
"We underscore the importance of a long-term solution to the instability in the eastern DRC," the State Department said in a statement. "A durable solution will require security sector reform in the DRC and political dialogue with Rwanda and Uganda and the other relevant states in the region to address the root causes of the crisis."
The statement said the United States was "gravely alarmed" by the deteriorating security situation in eastern DRC.
"We condemn the renewed military campaign by the M23 rebel group, which is an affront to the sovereignty and territorial integrity of the DRC and in violation of international law," the statement said. "We call for a cease-fire, and for the M23 to return Goma to DRC authorities ... ."
The U.N. Security Council voted unanimously to impose sanctions against M23 leaders. The resolution also demanded that the rebels withdraw from Goma immediately, and condemned all foreign support for the group.
M23 soldiers were considered part of the Congolese national army under a 2009 peace negotiation. However, they broke from the government army in April, complaining of poor conditions and lack of pay. One M23 commander, Bosco Ntaganda, is wanted by the International Criminal Court for war crimes, including child-soldier recruitment.