KAMPALA, Uganda — Heavy shelling and gunfire Monday broke a standoff between Congolese rebels on the outskirts of the eastern Congolese city of Goma and government soldiers backed by U.N. troops hunkered down inside.

Fears also rose of a military confrontation between Congo and neighbor Rwanda.

The Congolese government rejected an ultimatum made by rebels Sunday night to withdraw from Goma and accused Rwanda, which a U.N. panel has said has links to the March 23 rebels, of sending two battalions over the border to fight on their behalf. The government also accused Rwanda of firing a rocket that injured five civilians in Goma.

Rwanda has called the accusations “absolutely false and diversionary” and said it was “exercising restraint as of now,” according to a military spokesman, Brig. Gen. Joseph Nzabamwita.

Rwanda's military accused the Congolese army of bombing the nearby Rwandan border city of Gisenyi, killing one and injuring two others.

Amid the back and forth, Sekombi Katondolo, a radio journalist in Goma, said Congolese troops were aligning along the border and that fears of an escalation in the battle were high.

“It's really scary,” Katondolo said as fighting between government forces and rebel soldiers around Goma broke out Monday afternoon. “We knew it would happen, but we didn't think it would happen all of a sudden.”

Witnesses in Goma said heavy explosions pounded the central part of the city Monday afternoon and that there was a mass exodus of civilians from there.

Scores were believed to be injured in fighting Monday, along with several killed.

The United Nations expressed bewilderment and frustration at the rebel attack.

“They were able to bypass all of the positions we had,” said Hiroute Guebre-Sellasie, who heads the U.N. peacekeeping office in North Kivu province.