Defying expectations and calls to step down, Syria's President Bashar al-Assad was strengthening his grip on power as the conflict entered its eighth year.

His troops advanced in a ferocious assault on Eastern Ghouta, once the opposition's main bastion on the outskirts of the capital.

A war monitor said regime forces now control 70 percent of the area, splitting the remaining rebel territory into three shrinking pockets.

After a fierce air and ground assault, regime forces on Thursday captured Hammuriyeh town, in an isolated southern part of Ghouta.

The Syrian Observatory for Human Rights, a Britain-based monitor, said Hammuriyeh fell to regime forces after fighters from the Faylaq al-Rahman rebel faction withdrew.

The regime's advance into Hammuriyeh overnight had punched a corridor through the town into government-controlled territory.

Streams of women and children escaped through that corridor on Thursday, carrying plastic bags stuffed with clothes and pushing strollers piled high with suitcases and rugs.

They reached a regime checkpoint in Adra district, where ambulances and large green buses waited to take them to temporary shelters.

- 'Largest displacement' -

The Observatory said nearly 20,000 people fled the enclave in 24 hours before the flow stopped on Thursday evening.

It called the exodus "the largest displacement since the beginning of the assault on Ghouta."

The United Nations said it was trying to determine how many people have left the enclave.

"The UN has not observed the evacuations, but is visiting collective shelters where some of the evacuees are arriving," a UN spokesman said.

Eastern Ghouta had been the main rebel bastion on the outskirts of Damascus since 2012 and came under a devastating regime siege the following year.

That left the area's roughly 400,000 residents struggling to secure food and hospitals crippled by shortages of medicine and equipment.

On Thursday, a joint convoy of food supplies for some 26,000 people entered Douma, the largest town in Ghouta and part of a separate rebel-controlled pocket.

"This is just a little of what these families need," said the International Committee of the Red Cross, which delivered the aid alongside the Syrian Arab Red Crescent and the UN.

ICRC President Peter Maurer went with the convoy, the first time he had accompanied such an operation.

Twenty-five trucks were delivering food parcels and flour bags to hunger-stricken Douma residents when mortar rounds hit nearby.

Aid workers scrambled for cover, an AFP correspondent said, but were able to resume deliveries shortly afterwards.

- Divide-and-conquer -

Thursday's aid operation came after two consecutive days of medical evacuations from Douma, which saw dozens of civilians bussed out for treatment in Damascus.

Eastern Ghouta was in May 2017 designated a "de-escalation zone" -- an area where violence is supposed to ease, paving the way for humanitarian assistance and a nationwide truce.

But since February 18, Russian-backed government troops have pressed a ferocious air and ground assault on Ghouta that has brought most of the former opposition bastion under government control.

The United States accused Russia of being "complicit" in atrocities.

"The Russians made a deliberate choice not to restrain the Assad regime. Thus, the carnage in East(ern) Ghouta continues. Russia is morally complicit and responsible for Assad's atrocities," said Pentagon spokeswoman Dana White.

The remaining rebel-held areas have been cut off from each other, in what analyst Nawar Oliver said was part of a divide-and-conquer strategy.

"The summary is that the regime cut up Ghouta into three zones, to comfortably work on securing three different agreements," said Oliver of the Turkey-based Omran Institute.

Assad is determined to retake Ghouta in order to secure the capital, which is regularly battered by rockets and mortars fired from the adjacent rebel enclave.

Dozens have been killed in rebel fire on Damascus in recent weeks, including one on Thursday, said state news agency SANA.

The assault on Ghouta has left nearly 1,250 civilians dead, around a fifth of them children.

The UN has made repeated demands for an immediate ceasefire in Eastern Ghouta, but they have gone unheeded.

For the past seven years, international efforts to bring an end to the violence raging across Syria have consistently failed.

The conflict has drawn in world powers, with Russia backing Assad and Turkey supporting an array of rebels in Syria's north against the regime, jihadists, and Kurds.

Ankara and allied Syrian factions on January 20 launched a sweeping ground and air assault against the Kurdish-controlled enclave of Afrin in northwestern Syria.

The offensive has forced 30,000 civilians from Afrin city in the past 24 hours, the Observatory said.

The city is home to around 350,000 people and defended by the Kurdish People's Protection Units (YPG).