After cutting a swathe through government-held territory in northern and central Syria, the Islamist-led rebels were at the gates of the country's third largest city Homs, a war monitor said.

In a little over a week, the offensive has seen Assad's forces ousted from the major cities of Aleppo and Hama for the first time since the civil war began in 2011.

Should the rebels capture Homs, that would cut the seat of power in the capital Damascus from the Mediterranean coast, a key bastion of the Assad clan, which has ruled Syria for the past five decades.

Abu Mohammed al-Jolani, the leader of the Hayat Tahrir al-Sham (HTS) rebel group, said the goal of the offensive was to end Assad's rule.

"When we talk about objectives, the goal of the revolution remains the overthrow of this regime. It is our right to use all available means to achieve that goal," Jolani told CNN in an interview.

The rebel alliance is led by HTS, which is rooted in the Syrian branch of Al-Qaeda but has sought to soften its image in recent years.

- Sudden withdrawal -

In a bid to reinforce its defences around Homs, the government abandoned its territory in eastern Syria Friday, including the major city of Deir Ezzor, to free up troops for the battle.

As the army and its Iran-backed militia allies pulled out, Kurdish-led forces said they crossed the Euphrates and took control of the territory on the right bank of the river that had been vacated.

"In order to protect our people, our Deir Ezzor Military Council fighters were deployed in Deir Ezzor city and west of the Euphrates River," the Arab-majority council affiliated with the Kurdish-led Syrian Democratic Forces (SDF) said in a statement.

The Observatory said that government troops and their allies withdrew "suddenly" from the east and headed towards the oasis town of Palmyra on the desert road to Homs.

The Kurdish-led Syrian Democratic Forces expressed readiness for dialogue with the rebels, saying their offensive heralded a "new" political reality.

In the south, staff abandoned government buildings in the mainly Druze province of Sweida, while armed groups in neighbouring Daraa province took advantage of the army's disarray to seize a border crossing with Jordan, the Observatory said.

- Diplomacy -

The rebels launched their offensive on Wednesday of last week, the same day a ceasefire took effect in neighbouring Lebanon in the war between Israel and Hezbollah.

The Lebanese militant group has been an important Assad ally, alongside Russia and Iran.

Turkey, which has backed the opposition, said it would hold talks with Russia and Iran in Qatar this weekend on the latest developments.

Turkish President Recep Tayyip Erdogan said he hoped the rebel advance would go off "without incident".

Ahead of those talks, the top diplomats of Iran, Iraq and Syria met in Baghdad, where Syria's Bassam al-Sabbagh accused the government's enemies of seeking to "divide the region anew and redraw the political map".

Iran's Abbas Araghchi pledged to provide Assad's government with "whatever (support) is needed and requested".

Iraq's Fuad Hussein said security forces were "on high alert".

- Fear -

Fearing the rebels' advance, tens of thousands of members of Assad's Alawite minority began fleeing Homs on Thursday, residents and the Observatory said.

Homs was the scene of a years-long army siege of rebel-held neighbourhoods and deadly sectarian attacks in the early years of the civil war.

Syrians who were forced out of the country by the crackdown on the revolt were glued to their phones as they watched the developments unfold.

"We've been dreaming of this for more than a decade," said Yazan, a 39-year-old former activist who now lives in France.

Asked whether he was worried about HTS's Islamist agenda, he said: "It doesn't matter to me who is conducting this. The devil himself could be behind it. What people care about is who is going to liberate the country."

On the other side of the sectarian divide, there was fear among Homs's Alawite community.

Haidar, 37, who lives in an Alawite-majority neighbourhood, told AFP by telephone that "fear is the umbrella that covers Homs now".

"I've never seen this scene in my life. We are extremely afraid, we don't know what is happening."

On Friday, rebel fighters took the towns of Rastan and Talbisseh on the road from Hama to Homs, the Observatory said.

Footage posted on social media and verified by AFP showed rebels firing into the air as they drove through Talbisseh.

- 'Massive blow' -

The Observatory said 826 people, mostly combatants but also including 111 civilians, have been killed since the offensive began last week.

The United Nations said the violence has displaced 280,000 people, warning the numbers could swell to 1.5 million.

Many of the scenes witnessed in recent days would have been unimaginable earlier in the war.

In Hama, an AFP photographer saw residents set fire to a giant poster of Assad on the facade of city hall.

Online footage verified by AFP showed residents toppling a statue of Assad's father Hafez, under whose brutal rule the army carried out a massacre in the city in the 1980s.

The army admitted losing control of the city, though Defence Minister Ali Abbas insisted that its withdrawal was a "temporary tactical measure".

Aron Lund, a fellow of the Century International think tank, called the loss of Hama "a massive, massive blow to the Syrian government".

Should Assad lose Homs, it wouldn't mean the end of his rule, Lund said, but "with no secure route from Damascus to the coast, I'd say it's over as a credible state entity".