Explainer-Who is fighting in Syria and why?

Syrian rebels' capture of Aleppo from President Bashar al-Assad has brought the Syrian civil war back into focus, jolting frontlines that had been dormant for years with implications for the region and beyond.

WHAT IS GOING ON?

The rebels launched their surprise offensive on Nov. 26, attacking from areas to the north and northwest of Aleppo. They swept into the city on Nov. 29-30, forcing out government forces.

It is the first time control of the city has shifted since 2016, when government forces, backed by Russia and Iran, defeated rebels who had controlled Aleppo's eastern districts.

The rebels have pressed their advance in areas to the south and southwest of Aleppo, capturing territory in Hama province.

The government has vowed to fight back. Russia, which deployed its air force to Syria in 2015 to help Assad, is conducting airstrikes in support of the army.

It marks the most serious escalation of the conflict in years, adding to a toll which stands at hundreds of thousands dead since 2011, when the war mushroomed out of an Arab Spring uprising against Assad's rule. Since then, more than half the pre-war population of 23 million have been forced from their homes, with millions fleeing abroad as refugees.

WHO ARE THE REBELS?

The attack was initiated by Hayat Tahrir al-Sham. Formerly known as the Nusra Front, it was al Qaeda's official wing in the Syrian war until breaking ties in 2016.

HTS, led by Abu Mohammed al-Golani, has long been the dominant force in the Idlib region, part of an arc of the northwest where rebels maintained a foothold despite Assad's gains elsewhere.

The United States and Russia, Turkey and other states designate it a terrorist group.

Another rebel alliance has launched a separate offensive from areas north of Aleppo. These rebels are backed by Turkey and organised under the banner of the Syrian National Army.

WHY HAS THE CONFLICT FLARED UP NOW?

While peace has remained far off, the frontlines have not moved for years, with Syria partitioned into zones where foreign powers have troops on the ground.

Russia and Iran have sway over government-held areas, the single biggest chunk of Syria. The United States has forces in the northeast and east, backing the Kurdish-led Syrian Democratic Forces. Turkey has troops on the ground in the rebel-held northwest.

But the regional power balance has been shaken by more than a year of conflict pitting Israel against Iran and the militant groups it supports.

The Iran-backed Hezbollah, in particular, has suffered major blows during more than two months of war with Israel in Lebanon. Hezbollah, which ceased fire with Israel last week, helped Assad recover Aleppo in 2016.

U.S. National Security Advisor Jake Sullivan, speaking to CNN's "State of the Union", said it was no surprise that rebels would try to take advantage of a new situation, with the Syrian government's main backers - Iran, Russia and Hezbollah - distracted and weakened by conflicts, referring to regional conflicts and the Ukraine war.

A deal between Russia and Turkey had broadly stabilised the situation in the northwest since 2020. But Turkey has expressed growing frustration with Assad's failure to reach a deal with the opposition to end the conflict.

Turkish security officials have said while Ankara was working to stop rebel offensives, they were increasingly concerned about attacks by Syrian government forces on the rebels. Foreign Minister Hakan Fidan said Assad and the rebels needed to compromise.

One of Turkey's main concerns in Syria is the power held by the Kurdish-led groups which are allied to Washington but deemed terrorists by Ankara. Turkey's state-owned Anadolu news agency said the rebel Syrian National Army had taken the town of Tel Rifaat from the Kurdish YPG militia.

Both the Kremlin and Tehran have reiterated their support for the Syrian government.

IS THERE A PEACE PLAN?

The U.N. Security Council passed a resolution in 2015 aimed at ending the conflict, calling for a new constitution, U.N.-supervised elections and transparent and accountable governance.

Implementation has gone nowhere.

U.N. Syria envoy Geir Pedersen said the escalation showed a collective failure to bring about a political process and urged substantive negotiations to find a way out of the conflict.