Syrian President Bashar al-Assad, right, speaks with United Nations special envoy to Syria Staffan de Mistura in Damascus on Monday. (SANA via AP)

The U.N. representative to Syria on Tuesday formally proposed an “action plan” for a cease-fire in the besieged city of Aleppo that could serve as a step toward a broader solution to the ongoing civil war.

Speaking from the Syrian capital, Staffan de Mistura said such a confidence-building measure could be used “as a building block in the direction of a political solution” for a nearly four-year-old conflict that has killed an estimated 200,000 people.

“This is a new way to see and achieve what we hope to bring a de-escalation of violence,” he said at the end of a three-day visit to Syria. He said the government has responded with “constructive interest,” although Syrian rebels expressed doubts about the plan.

On Monday, de Mistura discussed the idea during a meeting in Damascus with President Bashar al-Assad, who deemed it “worth studying,” according to the state-run Syrian Arab News Agency.

Starting such a plan with Aleppo would have symbolic value. The city, once Syria’s commercial heart, has over the past two years come to embody the devastation wreaked by the conflict. Relatively moderate rebels have clung to Aleppo despite being gradually hemmed in by radical Islamist fighters and forces loyal to the government, which has intensified attacks on the city in recent weeks.

Amateur video purports to show Syrian rebels in Deraa and Aleppo firing rockets at the pro-government militia groups. The U.N. representative to Syria formally proposed an “action plan” for a cease-fire Aleppo on Tuesday. (Reuters)

Past cease-fires have been met with suspicion by rebel groups, which accuse the Assad regime of regularly breaking them.

Islam Alloush, a spokesman for the Islamic Front, an umbrella organization of rebel fighters, dismissed the proposal as mere “talk in the media.” He said it would be a successful initiative if it actually “stops the killings” and the bombs “delivered by Assad’s planes.”

De Mistura first proposed the idea of local cease-fires at the United Nations in New York last month. The freeze on fighting in certain war-torn areas of Syria would help humanitarian workers deliver aid to residents, he said. The truce also could serve as a steppingstone for a wider political settlement, which has proved elusive.

In his address Tuesday, de Mistura also referred to the extremist groups Islamic State and Jabhat al-Nusra, an al-Qaeda affiliate, as capitalizing on Syria’s instability, arguing that his plan could help avert “the real threat of terrorism.”

He also expressed shock at what he saw in Homs during a visit to the western city.

“Have you been to Homs? Have you seen the levels of destruction? Have you seen the levels of horrific destruction of a beautiful city? We don’t want that to happen in Aleppo,” he said.

In May, rebel forces evacuated Homs as part of a cease-fire deal after a brutal siege by regime forces that brought scores of residents to the brink of starvation. Many rebels regard such truces as surrender-or-starve agreements.

Those seeking a new approach appear to be driven by two new factors in the Syria equation — the rapid spread of the Islamic State and the U.S.-led intervention to counter it. Although the Assad regime seems to be winning against more moderate rebels, it has suffered significant setbacks against extremist groups, including the Islamic State’s takeover of government air bases and oil fields. At the same time, Turkey’s long-standing refusal to become directly involved in the Syrian conflict — a source of some comfort for Assad — appears to be softening.

Emile Hokayem, a Middle East security analyst at the London-based International Institute for Strategic Studies, said the cease-fire proposal for Aleppo could “lower levels of violence.” But referring to the fragmentation of rebel forces, he added that “these truces tend to crumble very quickly, and the idea could have a negative effect on the rebellion and fracture it even more.”

Hassan Hassan, a Syria analyst at the Abu Dhabi-based Delma Institute, said proposals such as de Mistura’s could be successful if properly implemented.

“With the right will from the international community, it is possible for such freezes to be part of a plan of consolidation of stability in areas under the control of either the regime or under moderate rebels,” he said.

The key, he said, is to “make sure areas stay outside extremists’ reach.”

Suzan Haidamous contributed to this report.