GOMA, Congo (AP) ― A rebel group believed to be backed by Rwanda seized the strategic, provincial capital of Goma in eastern Congo on Tuesday, home to more than 1 million people as well as an international airport in a development that threatens to spark a new, regional war, officials and witnesses said.

Explosions and machine-gun fire rocked the lakeside city as the M23 rebels pushed forward on two fronts: toward the city center and along the road that leads to Bukavu, another provincial capital which lies to the south. Civilians ran down sidewalks looking for cover and children shouted in alarm. A man clutched a thermos as he ran.

Thousands of residents fled across the border to Rwanda, the much-smaller nation to the east which is accused of funneling arms and recruits to the M23 rebels.

By early afternoon the gunfire had stopped and M23 soldiers marched down the potholed main boulevards, unimpeded. Their senior commanders, who the United Nations has accused of grave crimes including recruiting child soldiers, summary executions and rape, paraded around the town in all-terrain vehicles, waving to the thousands of people who left their barricaded houses to see them.

The United Nations peacekeepers, known by their acronym MONUSCO, were not helping the government forces during Tuesday’s battle because they do not have a mandate to engage the rebels, said Congolese military spokesman Olivier Hamuli, who expressed frustration over the lack of action by the peacekeepers.

“MONUSCO is keeping its defensive positions. They do not have the mandate to fight the M23. Unfortunately, the M23 did not obey the MONUSCO warnings and went past their positions (at the airport). We ask that the MONUSCO do more,” he said.

A U.N. spokesman said in New York said that the nearly 1,500 U.N. peacekeepers in Goma held their fire to avoid triggering a battle. The peacekeepers “cannot substitute for the efforts of national forces” in Congo, said spokesman Eduardo del Buey.

French Foreign Minister Laurent Fabius said Tuesday that the mandate of the U.N. forces in Congo “must be reviewed.” Speaking in Paris, he said it is “absurd” that the U.N. has 17,000 peacekeepers in all of Congo but they “couldn’t stop several hundred men” in Goma.

The French minister also said a rapprochement between Congo and Rwanda was critical to solving the crisis. Fabius said he’s been in touch with officials in both countries.

The rebels are believed to be backed by Rwanda, and to a smaller extent by Uganda, which are accused of equipping them with sophisticated arms, including night vision goggles and 120 mm mortars. Evidence is mounting of the involvement by the neighboring country and on Friday, the United Nations Group of Experts is expected to release its final report, detailing the role the neighboring nations played in the recruitment, financing and arming of the rebel movement, which was born in April.

Congolese government spokesman Lambert Mende said that Rwandan soldiers had crossed into Goma, hiking over footpaths across a volcano that looms between the two countries.

“Goma is in the process of being occupied by Rwanda,” said Mende, speaking from Congo’s distant capital of Kinshasa. “We have people who saw the Rwandan army traverse our frontier at the Nyamuragira volcano. They have occupied the airport and they are shooting inside the town. Our army is trying to riposte but this poses an enormous problem for them _ this is an urban center where hundreds of thousands of people live,” he said.

A Congolese colonel, who was at the frontline in Goma before the city fell, said that the soldiers he saw were Rwandan. Neither his claim nor Mende’s could be independently verified.

Congo’s President Joseph Kabila flew to Uganda Tuesday for talks with President Yoweri Museveni, a Ugandan government spokesman confirmed. The official spoke in Kampala on condition of anonymity because he was not authorized to divulge the information.

M23 rebel spokesman Col. Vianney Kazarama confirmed that they had taken the airport and the city. “We are now inside the city of Goma,” he said.

Goma, a city of low-lying buildings, many topped by rusted corrugated roofs, was last threatened by rebels in 2008 when fighters from the now-defunct National Congress for the Defense of the People, or CNDP, stopped just short of the city. Their backs to the wall, the Congolese government agreed to enter into talks with the CNDP and a year later, on March 23, 2009, a peace deal was negotiated calling for the CNDP to put down their arms in return for being integrated into the national army.

The peace deal fell apart this April, when up to 700 soldiers, most of them ex-CNDP members, defected from the army, claiming that the Congolese government had failed to uphold their end of the deal. Like in 2008, they again advanced toward Goma. This time, the city fell and the disastrous consequences for the population were already on display.

At a municipal hospital, 19-year-old Nene Lumbulumbu described how she was cleaning her house, when she was hit by a stray bullet in the torso. A father stood over his little girl’s bed, clutching an X-ray showing the bullet lodged in her chest. He had sent her to fetch water in this city where most live without running water. She was brought back by neighbors, after being hit by a bullet.

The hospital was treating children whose arms were sheared off by exploding shells, and teenagers paralyzed from the neck down. Hospital director Justin Lussy said the injuries were just the tip of the iceberg.

“Regional and international actors must now prevent this turning into a new regional war,” said the International Crisis Group in a statement. “The past week has shown history repeating itself in eastern Democratic Republic of Congo, with the same tragic consequences for civilians in the region.”

If the rebels succeed in taking Bukavu, it will mark the biggest gain in rebel territory since at least 2003, when Congo’s last war with its neighbors, including Rwanda and Uganda, ended.

Jean-Claude Bampa, who lives near the road to Sake, the first town on the drive to Bukavu, spoke on the telephone over loud gunfire in the background. “I can hear gunshots everywhere, it is all around my home,” he said on Tuesday morning. “We are stuck inside and are terrified. I pray this will be over soon.”