A look at the Syrian city of Aleppo and the UN proposal for a 'freeze' in hostilities there

FILE - In this Monday Sept. 24, 2012 file photo, a Free Syrian Army fighter, right, looks through a mirror which helps him see Syrian troops from the other side, as he takes his position with his comrade during fighting, at the old city of Aleppo city, Syria. Nearly four years since it began, Syria’s civil war has defied all diplomatic attempts to broker a peaceful resolution _ and the fierce fighting still goes on daily, though it has taken a back seat to the rampage in the region by Islamic State militants. (AP Photo/Hussein Malla, File) (The Associated Press)

FILE -- In This April 21, 2014, file photo, provided by the anti-government activist group Aleppo Media Center (AMC), which has been authenticated based on its contents and other AP reporting, shows a Syrian man holding a girl as he stands on the rubble of houses that were destroyed by Syrian government forces air strikes in Aleppo, Syria. Nearly four years since it began, Syria’s civil war has defied all diplomatic attempts to broker a peaceful resolution _ and the fierce fighting still goes on daily, though it has taken a back seat to the rampage in the region by Islamic State militants. (AP Photo/Aleppo Media Center AMC, File) (The Associated Press)

FILE - In this Nov. 10, 2014, file photo released by the Syrian official news agency SANA, Syrian President Bashar Assad, right, speaks with United Nations special envoy to Syria Staffan de Mistura in Damascus, Syria. Nearly four years since it began, Syria’s civil war has defied all diplomatic attempts to broker a peaceful resolution _ and the fierce fighting still goes on daily, though it has taken a back seat to the rampage in the region by Islamic State militants. (AP Photo/SANA, File) (The Associated Press)

Nearly four years since it began, Syria's civil war has defied all diplomatic attempts to broker a peaceful resolution. While the conflict has recently been overshadowed by the U.S.-led international battle against the Islamic State extremist group, Syria's war has continued its devastating march, with the death toll now at least 220,000 people.

The U.N. envoy to Syria, Staffan de Mistura, offered a small ray of hope this week, however, when he told the Security Council the Syrian government has agreed to suspend airstrikes and artillery shelling of the divided city of Aleppo for six weeks as part of a U.N.-proposed "freeze" in hostilities there.

Here are answers to a few key questions about the city and the prospects for a cease-fire in Aleppo:

— WHY DOES ALEPPO MATTER?

The city holds strategic as well as symbolic value. Before the war, it was Syria's largest city as well as its commercial capital, and was home to a population that represented the mosaic of faiths and ethnicities that make the country so diverse: Arab Sunni and Shiite Muslims, Kurds, Alawites and Christians of various stripes — Armenians, Assyrian, Orthodox, Catholic.

De Mistura says he is focusing his efforts on Aleppo because it "is a symbolic microcosm of all of Syria, because it has the highest number of displaced people, because it has seen two years of suffering, because while the government and the opposition continue being involved in heavy fighting between them, ISIS is only 20 miles away."

The U.N. envoy also envisions a local truce in Aleppo as the first step toward a wider easing of hostilities.

— WHAT FORCES ARE IN ALEPPO?

The rebel forces run the full spectrum of the armed opposition, from U.S.-backed mainstream groups to Islamic extremists. The largest and most prominent faction in the city is the Islamic Front, a coalition of seven conservative Islamic groups. The Islamic Front banded together late last year with four other significant rebel groups, including U.S.-backed mainstream brigades, to form the Levant Front in the hopes of better organizing the opposition fight in the city. The al-Qaida-affiliated Nusra Front and another extremist group led by Chechen fighters are also present in Aleppo.

On the government side, the Syrian military is joined by pro-government militias and fighters from the Lebanese Shiite militant Hezbollah group. The government has largely relied on its superior firepower, particularly its warplanes and helicopters, to inflict massive damage on rebel-held areas of the city.

— WHAT IS THE STATE OF PLAY IN ALEPPO NOW?

Aleppo has been carved in two since opposition fighters launched an assault on the city in mid-2012, leaving the eastern half in rebel hands and the western half with the government. The fighting since then has settled into a bloody grind of pitched street battles and devastating government air raids that have reduced entire neighborhoods to rubble. Civilians in rebel-controlled areas live in desperate conditions and under constant threat of aerial or artillery bombardment, while on the government side mortars and homemade rockets fired from opposition areas cause frequent casualties.

The rebel position has grown precarious in recent months as opposition fighters struggle to manage a two-front war against the government and the Islamic State group. Forces loyal to Assad have advanced along the northeastern edge of the city and now threaten to cut vital opposition supply lines to the countryside north of Aleppo. On Tuesday, government troops seized several villages just north of the city in a surprise attack, only to be expelled from most of them in heavy fighting. But if the offensive succeeds, government forces will have encircled the rebel-held half of the city. Activists and rebel fighters fear the government could then employ the slow-burn siege strategy it used last year to strangle rebel holdouts in Homs, Syria's third-largest city, into submission. Assad's forces have also used blockades, which have prevented food and medicine from reaching thousands of civilians as well as fighters, to bring several rebellious suburbs of Damascus to heel.

The other source of pressure on the opposition comes from the Islamic State group. The extremist militants have pushed to within some 30 kilometers (20 miles) northeast of the city, forcing the rebels to commit manpower and resources to try to halt the IS advance.

— WHAT ARE THE PROSPECTS OF A "FREEZE" IN FIGHTING ACTUALLY HAPPENING?

De Mistura and his team have been meeting with international leaders, Syrian government officials in Damascus as well as Syrian opposition and rebel leaders in southern Turkey to dry to drum up support for his truce proposal. Securing a commitment from the government to suspend airstrikes and artillery is step forward, but hurdles remain. The U.N. envoy still needs the armed opposition's support for the plan, which includes a request for them to suspend rocket and mortar fire in the same period — a difficult task with the multitude of rebel factions present in Aleppo. The government, meanwhile, has time and again publicly agreed to international peace efforts while simultaneously ignoring the commitments it has made under them.

One of the chief hurdles, de Mistura said last month, is the lack of trust, and "that is causing a lot of problems because no one wants to move first. And there needs to be simply one thing: a freeze."

— WHAT ARE THE STEPS AHEAD?

De Mistura's plan for now calls for a suspension in heavy weapons fire, and a full "freeze" in hostilities in one district of Aleppo. The idea is to bring peace to that one district, and then build out from there, a neighborhood at a time. One U.N. diplomat said de Mistura indicated the "freeze" plan would first be attempted in Salaheddine, a densely populated and contested area in central Aleppo. The diplomat spoke on condition of anonymity in line with regulations.

The envoy is expected to head to Damascus soon, where he will announce a date for the start of the truce. De Mistura also plans to send a preparatory team to Aleppo as soon as possible.

On Tuesday, he told reporters he was aware of all the difficulties.

"Let's be frank. I have no illusions," de Mistura said.