Syrian rebel fighters have evacuated one enclave in eastern Ghouta and reached a Russian-brokered deal to leave another, enabling Syrian government forces to regain control over most of the embattled Damascus suburb, observers say.

The developments on March 23 brought Syrian President Bashar al-Assad close to his biggest victory since driving armed rebels out of Aleppo in December 2016, though his opposition still has a few remaining strongholds in northwestern and southwestern Syria.

A devastating Russia-backed government assault since February 18 on eastern Ghouta has killed more than 1,600 civilians but has brought 90 percent of the Damascus suburb back under government control, the Syrian Observatory for Human Rights said.

Retaking eastern Ghouta, a sprawling semirural area that had escaped government control since 2012, became a priority this year for Assad after a string of gains by the Syrian Army elsewhere around the country.

At the start of the offensive, the United Nations estimated 400,000 people were trapped inside the besieged area without access to food or medicine.

The Russia-backed Syrian campaign succeeded in splintering the territory held by fractured rebel groups into three shrinking pockets, each held by different rebel factions.

Damascus and its ally Moscow then implemented a "leave or die" strategy like the one that enabled them to prevail after a similarly bloody battle in Aleppo.

The killing of hundreds of civilians through intense bombardments within the span of a month provoked an international outcry and global condemnation at the United Nations, but Western efforts to impose a cease-fire that might save civilian lives never took hold.

Syrian state television said on March 23 the town of Harasta had been "emptied" of rebels after an evacuation deal that saw fighters from the Islamist group Ahrar al-Sham and their families bused to northwestern Idlib Province, one of the last strongholds for the rebels.

Syrian Army soldiers fired guns in the air in celebration as the last groups of rebels in Harasta boarded buses.

Assad's forces looked set to expand their control further within days after another Islamist group, Faylaq al-Rahman, struck a deal with the Russian military to evacuate starting on March 24.

The agreement will see a southern stretch of territory that includes the towns of Zamalka, Arbin, and Ain Tarma cleared of rebels, while talks are under way over clearing a third and final pocket of rebels, around Ghouta's main town of Douma.

The withdrawal deal came after the Syrian war monitor and the White Helmets rescue group said Russian air strikes using "incendiary munitions" hit the town of Arbin in the last rebel-held enclave late on March 22. Russia denied it was involved in such strikes.

Under the strategy employed by Russia and Syria, each one of the three rebel-held pockets came under intense bombardment that not only killed hundreds but forced people to seek shelter and live underground.

The strategy in each case succeeded in forcing rebel fighters to accept Russia's offer to give them a safe exit if they agreed to leave.

It is a tactic Assad and his Russian sponsors used successfully in Aleppo and elsewhere since Russia intervened in the seven-year civil war in 2015.

Harasta's evacuation -- which began on March 22 -- was the first such deal in eastern Ghouta negotiated by Russia. Syrian state media reported more than 4,500 people, including over 1,400 fighters, left the town.

"Our situation in Harasta was very tragic," Abu Muhammad, a civilian, told AFP. "We couldn't live aboveground because of the heavy bombardment... Some children stayed in basements for four months with no food. We couldn't even find fodder to eat."

Following the same pattern, Faylaq al-Rahman on March 22 sealed a deal for some 7,000 people to leave the enclave the rebel group controlled, which was experiencing famine and the outbreak of disease under the bombardment.

After being hit with what appeared to be incendiary munitions in Douma, the last rebel-held enclave, the Jaish al-Islam rebel group also opened negotiations with Russia on March 23.

The UN estimates more than 50,000 people have fled besieged areas of eastern Ghouta in the past two weeks.

Russia is claiming that more than 100,000 civilians have been evacuated from the area with its assistance since it started imposing daily "humanitarian pauses" in Ghouta, the Russian state-run TASS news agency quoted Russian official Yuri Yevtushenko as saying on March 23.

With reporting by AFP, dpa, and Reuters