For two weeks, a battle has raged in Aleppo, generating tragic images of injured civilians amid the rubble.

The city — once the country's most populous, and a commercial hub — is a key prize in the civil war. For four years, it has been divided between government and rebel forces and was in effect a military stalemate.

Russia is among the supporters of Syrian president Bashar Assad, while the U.S. supports rebel forces. They were talking to try to find a way to calm the violence in Syria, but the negotiations collapsed this week.

With a diplomatic solution a distant prospect, attention has shifted to the battlefield, and the possibility that the Syrian military could capture Aleppo, the last major city where rebels have a real presence.

"The battle of Aleppo is the culmination of many years of fighting," says analyst Jennifer Cafarella with the Washington, D.C.-based Institute for the Study of War. For four years, she says, Assad's forces and regional allies fighting alongside them have planned to recapture the whole of Aleppo city and the surrounding countryside.

The last two weeks have seen significant advances around the city by pro-regime ground forces. This is largely because Russia has stepped up its air support of Assad's offensive. Residents of eastern Aleppo describe bombs which destroy buildings down to the basement, and incendiary weapons.

More than 300 civilians have died in the last two weeks on the rebel side of the lines, according to the opposition-leaning Syrian Observatory for Human Rights. The aid group Doctors Without Borders and U.N. agencies also count hundreds of civilian deaths there. The Observatory tallied 15 civilian deaths on the government-held side of the city, killed by rebel shelling.

"The opposition and the civilian population have not been able to withstand it," says Cafarella of the Russian-led bombardment.

But even with this air campaign, pro-Assad ground forces will probably not immediately be able to storm the urban terrain held by the rebels. That would require street-to-street fighting against thousands of rebel forces who have had years to dig in, even building tunnels.

But the regime has managed to cut off supply lines from Turkey to the rebel area. That means food and medical aid can't get to the opposition enclave, and neither can ammunition. This siege has been in place about a month now.

Residents say they fear either that Assad's forces will advance, retake eastern Aleppo and massacre civilians — or that they will maintain a siege which will, maybe after months or years, force a surrender.

So how are the rebels responding? They've staged some offensives elsewhere, apparently as diversionary tactics.

And Cafarella notes, "They also do seek to break the siege, actually, and to reopen either the northern or the southern entrance to the city."

NPR reached a commander with one rebel faction, who wouldn't give his name because he's not an official spokesman.

"The battle to open a path is first," he says. "To make a path and break the siege for our people in Aleppo city."

He complains that countries supporting the rebels – including the U.S. but also Turkey and the Gulf countries – have refused to supply them with anti-aircraft weapons. The rebels have long sought the arms but the U.S. and other allies fear they would fall into the hands of extremist factions, including a group with links to al-Qaeda.

"Really, if we got anti-aircraft weapons to fight the regime, God willing, it would be defeated," he said.

For the commander, this is existential. After five years of war, this chunk of eastern Aleppo city is the last significant bit of urban territory controlled by rebels.

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