GOMA, Congo (AP) — Rebels believed to be backed by Rwanda fired mortars and machine guns Monday on the outskirts of the provincial capital of Goma, threatening to capture one of the largest cities in eastern Congo in a development that could drag this giant Central African nation back into war.

The gunfire and explosions erupted in the early afternoon, with shells landing as far away as the international airport and near a United Nations position, causing flights to be rerouted and prompting the United Nations to evacuate most of its employees, according to U.N. officials.

The violence erupted just hours after the M23 rebels said they were halting fighting to negotiate with the government of Congo. But government spokesman Lambert Mende told The Associated Press by phone that negotiations are out of the question, saying Congo will not give in to the "blackmail" of a Rwandan-backed group.

"We refuse to enter into negotiations with M23. Because it's Rwanda, not the M23, that is responsible," said Mende. "If Goma falls, it's going to create a whole other set of problems. We refuse systematically to speak to them (M23). Because if we do, it would be a way to wash away Rwanda's responsibility."

Congo and Rwanda have already fought two wars, the most recent of which ended in 2003 after lasting nearly six years. On Monday, both nations accused the other of firing mortars across the narrow border which runs on one side of Goma, a city of 1 million which is the economic heart of Congo's mineral rich region.

Rwandan military spokesman Gen. Joseph Nzabamwita said that Congolese shells had fallen on the Rwandan side, while Mende said that a mortar fired from Rwanda landed in the Birere neighborhood near the airport in Goma and wounded at least five people.

As tanks rumbled by, civilians including young children could be seen running to safety, seeking shelter in huts and behind ledges along the road where the two sides were battling.

Earlier, M23 rebel spokesman Col. Vianney Kazarama told the AP that his men were on their way back to Kibumba, 30 kilometers (18 miles) north in order to give proposed talks a chance. But on Monday afternoon, Kazarama blamed Congo for renewed hostilities and once again vowed that M23 would take Goma.

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"The army provoked us. They have fired on our men ... We are going to take Goma tonight," he said.

On Saturday, U.N. peacekeeping chief Herve Ladsous said the rebels are very well-equipped, including with night-vision equipment allowing them to fight at night. The new equipment, including the goggles as well as 120 mm mortars, are being provided by Rwanda, which also sent several battalions of fighters, according to the International Crisis Group.

"The situation in Goma is extremely tense," said U.N. spokesman Kieran Dwyer in a statement Sunday. "There is a real threat that the city could fall into the M23's hands and/or be seriously destabilized as a result of the fighting."

At U.N. Headquarters in New York on Monday, French Ambassador Girard Araud said he would be introducing a Security Council resolution imposing sanctions on the M23 rebel leadership.

On Monday, the rebels lofted mortar shells that landed near U.N. peacekeeping troops at the airport, Dwyer said in an update, adding that "We are not able to confirm fighting in Goma or that the M23 has reached Goma at this stage." He also said the U.N. could not confirm reports of firing from Congo into Rwanda.

M23 began when several hundred men believed to be led by Gen. Bosco Ntaganda — who is wanted by the International Criminal Court —defected from the Congo army in April. Congo analyst Jason Stearns, a former member of the United Nations Group of Experts, said on his blog that the group is now believed to be composed of 2,500 to 3,000 men.

The situation mirrors events in 2008, when a now-defunct rebel group known as the National Congress for the Defense of the People, or CNDP, advanced to the edge of Goma as Congolese soldiers dropped their weapons and ran. That group, which was financially and militarily backed by Rwanda, stopped just short of taking the city. The rapid advance forced the Congo government to enter into negotiations with the rebels.

The peace deal brokered on March 23, 2009, called for CNDP fighters to be integrated into the national army.

Tellingly, the new rebel group's name comes from the date of those peace accords, which the M23 says were never fully implemented by the government. M23 fighters include former members of the CNDP.

The M23 rebels told the Congo government to make a declaration on state TV and radio announcing the start of negotiations, but did not state what they hope to achieve in talks. In a statement released on Monday, the M23 also called for the immediate demilitarization of the city and the airport in Goma, and for the opening of the border at the town of Bunagana within the next 48 hours.

The U.N. Group of Experts says M23 is backed by neighboring Rwanda, which the Rwandan government of President Paul Kagame denies. Uganda is also accused of backing M23 by the U.N. experts in a report that was leaked to the media last month, and Uganda also denies the allegations.

Observers say it is in Rwanda's interest to exert influence over areas of eastern Congo bordering Rwanda, where Hutus fled after perpetrating the 1994 genocide inside Rwanda against the country's Tutsi minority. Exerting influence would enable Rwanda to maintain a buffer zone and to exploit the trade and trafficking of minerals in eastern Congo, say experts including those from the International Crisis Group.

Over the weekend, U.N. Secretary-General Ban Ki-moon called Kagame to ask him to intervene and stop the M23 offensive, according to a statement issued at U.N headquarters in New York.

Ban also spoke Sunday to Uganda's President Museveni, according to Dwyer. Museveni, in his capacity as chairperson of the International Conference of the Great Lakes Region, indicated that he had spoken to the M23 rebels and called for calm.

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Callimachi contributed from Dakar, Senegal. Associated Press writer Saleh Mwanamilongo in Kinshasa, Congo, and Peter James Spielmann at the United Nations contributed to this story.