Rebel forces pushed their offensive in Syria further south early Friday, closing in on the central city of Homs as humanitarian groups sounded the alarm on the escalating conditions for civilians trapped in the fighting.

It comes as Russia's embassy in Damascus also urged its nationals to leave Syria on commercial flights, the TASS state news agency reported.

A day earlier, pro-government forces were ousted from Hama, Syria's fourth-largest city. That gave the insurgents a major new victory after a lightning advance across northern Syria while dealing another blow to President Bashar al-Assad and his Russian and Iranian allies.

If rebel groups are successful in seizing the key crossroads city of Homs, they would cut off Syria's capital Damascus from the coast, a longtime redoubt of Assad's minority Alawite sect and home to Russia's naval and air bases.

"The battle of Homs is the mother of all battles and will decide who will rule Syria," said Rami Abdulrahman, chief of the U.K.-based Syrian Observatory for Human Rights (SOHR), an opposition war monitor.

In a further setback for Assad, a U.S.-backed alliance led by Syrian Kurdish fighters has taken Deir Al-Zor, the government's main foothold in the vast desert east of the country, sources told Reuters on Friday.

It was the third major city, after Aleppo and Hama in the northwest and centre, to fall out of Assad's control in a week.

WATCH | 'High time' for parties to seek resolution to conflict, UN says: 

See the streets of Hama after rebels captured the Syrian city

Damaged army vehicles rested on roadsides in Hama, Syria, on Friday after Hayat Tahrir Al-Sham rebels captured the city.

The violence has already displaced some 280,000 people since it erupted in late November, according to the latest United Nations figures.

"If the situation continues evolving [at the same] ... pace, we're expecting collectively around 1.5 million people that will be displaced and will be requiring our support," said Samer AbdelJaber, head of the World Food Programme's Emergency Coordination, Strategic Analysis and Humanitarian Diplomacy arm, told reporters in Geneva.

After years locked behind frozen front lines, the insurgents have burst out of their northwestern Idlib bastion to achieve the swiftest battlefield advance by either side since a street uprising against Assad mushroomed into civil war 13 years ago.

Rebel fighters led by Hayat Tahrir al-Sham (HTS), which has been listed as a terrorist entity by Canada since 2013, said they had taken over the towns of Talbiseh and Rastan. The group, which broke from al-Qaeda in 2016, says it poses no threat to the West and has spent years trying to moderate its image, presenting itself as a viable alternative to Assad.

Children 'paying the heaviest price'

Save the Children Syria is sounding the alarm on the harm to children caught in the fighting, who were already reeling from the effects of 13 years of war, subsequent humanitarian and economic crises and a devastating earthquake that affected nearly 40 per cent of its population.

The U.K.-based organization said at least 69 civilians, including 26 children and 11 women, were killed in northwest Syria in the past week. At least 228 others were reportedly injured, including 88 children and 53 women.

Rasha Muhrez, Save the Children's Syria response director, said she received reports of families fleeing the fighting, with nowhere to go, sleeping in the streets for several days. Muhrez said some schools have been transformed into shelters but cannot accommodate enough people.

"It's winter, it's the middle of the school year.… In the end, children are paying the heaviest price for it," Muhrez told CBC News Friday from Amman, Jordan.

WATCH | Civilians at risk as Syria violence flares, UN chief says: 

Civilians at risk as Syria violence flares, UN chief says

United Nations Secretary General António Guterres says after years of conflict in Syria, it is 'high time' for all parties to engage seriously and chart a path to resolving the long-running crisis.

Some 120 schools in the northeast town of Raqqa are being used to accommodate 35,000 individuals, according to the organization.

Aid agencies say they had only raised less than a third of the $4 billion needed to run programs in 2024 before the fighting erupted last week.

Earlier this month, the UN humanitarian office said it had had to cut food rations in Syria by up to 80 per cent due to insufficient funds.

"The situation in Syria was not easy before this escalation, so we're looking at a crisis on top of crisis. And that's why we're really emphasizing the urgent need for funding," AbdelJaber said.

Jordan closes Syria border

On Friday, Jordan also closed its only passenger and commercial border crossing into Syria — known as the Jaber crossing — Jordan's interior ministry said. Jordanians and Jordanian trucks would be allowed to return via the crossing, on the Jordanian side, but no one would be allowed to cross into Syria.

A Syrian army source told Reuters that armed groups who infiltrated the crossing attacked Syrian army posts stationed at its Nassib border into Jordan.

Muhrez said the closure of the border will likely have a big impact, although it's unclear how it will affect specific aid delivery. She added that the crossing has been critical in delivering aid and humanitarian assistance to Syria and Lebanon.

Homs empties as residents flee

A resident of Homs said the offices of Syria's main security branches there emptied on Friday morning, with members leaving the city.

SOHR said thousands of people had begun fleeing from Homs on Thursday night, heading toward the Mediterranean coastal regions of Latakia and Tartus.

Wasim Marouh, a Homs resident who decided not to leave, said most of its main commercial streets were empty while pro-government militia groups patrolled the streets.

A rebel operations room, meanwhile, urged Homs residents in an online post to rise up, saying: "Your time has come."

WATCH | Syrian army withdraws from Hama Thursday:

Syrian army withdraws from Hama after anti-government insurgents grab control

Syrian rebels captured the city of Hama, a major victory in their advance across northern Syria and a devastating new blow to President Bashar al-Assad and his Russian and Iranian allies.

Russian bombing overnight destroyed the Rastan bridge along the M5 highway, the main route to Homs, to prevent rebels from using it to advance, a Syrian army officer told Reuters, adding that government forces were bringing reinforcements to positions around Homs.

Assad relied heavily on Russian and Iranian military backing during the most intense years of the civil war, helping him to claw back most territory and Syria's largest cities before front lines hardened in 2020.

But Russia has been focused on its invasion of Ukraine since 2022. And many in the top leadership of Hezbollah, the most powerful Iran-aligned militia force, were killed by Israel over the past two months.

A senior Iranian official said Tehran will likely need to send military equipment, missiles and drones to Syria in addition to sending more advisers and deploying forces. He said Tehran is currently providing intelligence and satellite support.

Hezbollah, meanwhile, sent a small number of "supervising forces" from Lebanon to Syria overnight to help prevent anti-government fighters from seizing Homs, two senior Lebanese security sources told Reuters.