A rebel group created just seven months ago seized large parts of the strategic provincial capital of Goma, home to more than 1 million people in eastern Congo, and its international airport on Tuesday, according to a rebel spokesman, residents and eyewitnesses.

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 and roads looking for cover and children shouted in alarm as gunfire crackled in the distance. A man clutched a thermos as he ran. A white tank with UN emblazoned on its side rolled down a Goma street, passing a Congolese army tank.

The U.N. 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 was frustrated over the lack of action by the peacekeeepers.

"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.

The rebels are believed to be backed by neighboring Rwanda, which is accused of equipping them with sophisticated arms, including night vision goggles and 120 mm mortars, according to reports by the United Nations. One Congolese military commander said Rwandans were among the rebels his men were battling. The claim could not be immediately independently verified.

"We already took the airport and part of the city," said rebel spokesman Col. Vianney Kazarama, reached by telephone on Tuesday. "We are now inside the city of Goma."

An official at the United Nations peacekeeping base in Goma, who insisted upon anonymity because she was not authorized to speak to the press, confirmed that the airport had fallen. A factory owner whose business faces the airport said he saw the rebels go onto the tarmac. By late morning, the rebels had entered the city center, though it remained unclear if they controlled the sprawling regional capital.

Goma 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 Goma, after intense international pressure. 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. They charged that they were not properly paid and equipped and that the government has systematically discriminated against ethnic Tutsis, which make-up the majority of their ranks.

Although M23 is tapping in to long-held grievances regarding the marginalization of Tutsis in Congo, analysts and country experts say the real reason for the rebellion is over control of Congo's vast mineral riches, a good chunk of which is concentrated in North Kivu province, where Goma is located, as well as neighboring South Kivu, of which Bukavu is the capital.

The United Nations and humanitarian groups warned that the fall of Goma could result in a humanitarian catastrophe. The city is home to several large camps of internally displaced people and already one camp housing over 60,000 refugees was broken up by the advance of the rebel army.

Sasha Lezhnev, policy analyst at the Washington-based Enough Project, said the international community and specifically U.N. peacekeepers need to do more to stop the rebel advance.

"(The fall of Goma) would represent a sharp escalation in the conflict, and really highlight the need for the international community to play a bigger role in resolving this conflict ... It could amount to a huge emergency," he said.

Germany, which is a member of the U.N. Security Council, called on the rebels to halt their military action immediately. Foreign Minister Guido Westerwelle said in a statement that he called on Congo's neighbors, a reference to Rwanda and Uganda which are accused of backing M23, to not do anything to worsen the crisis. "I expect of Congo's neighboring states that they refrain from doing anything which further exacerbates the situation," Westerwelle said.

Several in-depth investigations by the United Nations Group of Experts have shown that M23 is being propped up by Rwanda, which is providing them with arms as well as with soldiers.

Over the weekend, U.N. Secretary-General Ban Ki-Moon called Rwandan President Paul Kagame and asked him to call the M23 leaders and ask them to stop their advance, according to a statement issued at U.N. headquarters in New York. In January, Rwanda will assume a seat on the United Nations Security Council, creating a diplomatic minefield in light of what is happening in Goma.

On Tuesday, a colonel in the Congolese army who was in Goma fighting the rebels said by telephone that the soldiers he is fighting are Rwandan. He requested anonymity because he is not authorized to speak to the press.

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 ended.

Fidel Bafilemba, a researcher for the Washington-based Enough Project who lives near the road to Sake, the first town on the drive to Bukavu, said: "The road to Sake seems to be controlled by the M23. A lot of people are fleeing toward the center of town, carrying mattresses, belongings on their heads."

Another resident living near the strategic road, Jean-Claude Bampa, 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."

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Callimachi reported from Dakar, Senegal.