A rebel group now in control of a key city in eastern Democratic Republic of the Congo announced Wednesday that it plans to "liberate" the entire country.

A spokesman for the rebel M23 group said its successful operation to take Goma on Tuesday, the culmination of several days of heavy fighting against government forces, was just the beginning.

"We will push on to Bukavu, then Kisangani, and finally take Kinshasa and overthrow the government," Lt. Col. Vianney Kazarama said to enthusiastic cheers from a crowd of several hundred at Goma's stadium, according to a Congolese reporter there.

Kazarama said that once the rebels overthrow the government, they will call for elections.

Kinshasa, the country's capital, lies nearly 1,000 miles to the west of Goma, the capital of North Kivu province.

On Wednesday, M23 consolidated its control of Goma, calling on policemen and the remaining government soldiers in town to integrate with the rebel group.

Activity started to return to normal, with shop owners restarting their businesses -- some after closing their doors for several days because of the tense situation. M23 has called on schools and banks to reopen as early as Thursday.

The front line, according to the rebel fighters, has moved to the west of Goma, to Mugunga.

The U.N. special representative for DR Congo, Roger Meece, briefed the U.N. Security Council on the "extremely serious crisis" in North Kivu by video link from Kinshasa on Wednesday.

"The security situation in North Kivu as well as in other areas in the east of Congo has seriously deteriorated over recent days, and this is compounded by a vast humanitarian crisis affecting hundreds of thousands of Congolese citizens," Meece said.

He also warned that violent protests against the Congolese authorities, as well as U.N. facilities and its staff, might spread to other parts of the country.

The M23's occupation of Goma and another nearby town, Sake, had been in the face of "significant resistance" by the Congolese army, supported by MONUSCO, the U.N. mission in DR Congo, he said.

The rebels were initially held in check but were able to come back with "sophisticated tactics," he said, with their capabilities apparently bolstered by "external support."

"As we have seen in other areas occupied by M23, the occupation of Goma also poses a major risk of increased serious human rights violations, including killings and forced conscription of civilians, including minors," Meece said.

The United Nations has already received numerous reports of targeted summary executions of those who try to stand in the rebels' way, he said, and there is an increased threat of sexual violence.

Meece said he believed it was possible to roll back the rebels' gains and restore stability, but only with a strong commitment from the international community.

The U.N. Security Council backed a resolution Tuesday calling for sanctions against the rebel leadership and "those providing external support to the M23."

The United Nations and some donor countries have accused neighboring Rwanda of backing the M23 by providing it with arms, support and even soldiers. Rwandan President Paul Kagame has repeatedly denied the allegation.

CNN iReporter Aimee Manimani, an aid worker with humanitarian group World Vision, told of her grief as renewed violence in Gisenye, just outside Goma, forced her to flee across the border to Rwanda.

"Yesterday, when I heard the sounds of the bombs and mortar, I just got depressed. I started crying. I couldn't control myself. I had to leave home again," she said.