How a Syrian rebel group pulled off its stunning seizure of Aleppo

Rebel militias in northern Syria have continued to make lightning-fast territorial gains against the government of President Bashar al-Assad, as videos posted to social media show fighting raging in several of the beleaguered nation's major cities, including the capital Damascus.

A well-armed rebel group, called Hayat Tahrir al-Sham (HTS), stunned government forces this week as its fighters seized control of the major northern city of Aleppo, Syria's longtime commercial capital that was fought over for years in the 2010s.

The group's men have since launched several incursions further south towards other major population centers.

Following years of military stalemate, in which a low-intensity conflict had persisted primarily in the country's northwestern region of Idlib, these developments appear to have upended long-held calculations about Syria's 13-year civil war, which began during the 2011 Arab Spring uprisings.

Assad's forces hamstrung by setbacks for Russia and Iran-backed militias

For years, Assad's forces had leaned on support from Russia and Iran-backed militias to grind down various rebel factions that had sprung up to oppose his rule. Until this past week, Assad maintained control over much of the country, albeit tenuously in some areas.

But Hezbollah, the leading Iran-backed militia in the region, has weakened as a result of Israeli actions in Lebanon as well as its airstrikes on Hezbollah-linked targets inside Syria itself. The militant group's decline has highlighted the limitations of Assad's forces, according to Robert Ford, a former U.S. ambassador to Syria who was first posted there just months before the civil war began in 2011. Russia has also been transferring weaponry and personnel away from Syria, directing some of those resources instead to the war in Ukraine.

"What this shows is that the Assad military forces were hard — I mean, they could inflict a lot of casualties, especially on civilian targets, but they were very brittle," says Ford. "The regime is obviously very weak, and its external supporters are much weaker than they were a couple of years ago, and, frankly, much weaker than almost anybody expected."

Early Saturday, the Syrian Observatory for Human Rights monitoring group said rebels had entered western Aleppo, and videos and photos shared online showed armed men celebrating in the city's main squares and posing in front of the city's ancient citadel.

HTS, which was once pushed out to rural areas along the border with Turkey, has for years received Turkish funding and military training. Now, in just three days, they have stormed deep into the country, and say they are now targeting the takeover of the government stronghold, Hama, in the country's center.

For some time, Turkey seemed to have prohibited HTS, as a recipient of their support, from launching this kind of attack on government territory, says Dareen Khalifa, a senior advisor at the International Crisis Group who focuses her research and advocacy work on peacemaking. But, she adds, the Turks may recently have gone through a change of heart given the regional changes.

"While they remain, of course, concerned about the fallout, I think they're way less concerned than they were a few years back," she says of the Turkish government. "That has been a key enabler, I think, in this offensive."

The Syrian military has said it is regrouping to launch a counteroffensive, but analysts say that may prove difficult.

"The combination of sheer dependence on a weakened Iran and rampant corruption within Syrian army ranks have meant the Syrian army has very little capacity to hold ground," says Lina Khatib, an associate fellow at the Middle East and North African program at British think tank Chatham House. "The regime had taken for granted that it had basically won the war, and had become complacent."

Syrian state media reported that — despite what appeared to be a rout of government forces from Aleppo — Russia had continued to provide the Syrian military with air support, as it did for many years during the civil war's most brutal years, when Russian aircraft dropped barrel bombs on urban centers with dense civilian populations.

President Assad's whereabouts were unknown late Saturday, ahead of what the news channel Al Arabiya reported was a scheduled visit from Iran's foreign minister Sunday.

The U.S. has not commented publicly on developments in the region, despite the long-standing presence of American troops in the country's northeast, where the U.S. military established bases in the wake of the campaign against ISIS.

"The world has taken its eye off the ball," says Myles Caggins, a senior fellow at the New Lines Institute think tank and a former spokesperson for the anti-ISIS military coalition that U.S. led in northeast Syria. "Between Russia, Turkey and Iran, they have mostly established a stalemate of instability across Syria, from east to west, north to south, with areas that are overlapping — of different competing interests."

Caggins says he's concerned about newly ungoverned and unstable territories helping to strengthen radical groups.

"We're further away from a solution today than we were two days ago," he said. "Ultimately, the world needs to put its arms around in Syria and find out a final settlement."

NPR's Ruth Sherlock contributed reporting.