Syria's President Bashar al-Assad on Monday branded an Islamist-led offensive that has captured swathes of territory a bid to redraw the map of the region in line with US interests.

His comments, made in a phonecall with Iranian President Masoud Pezeshkian, came as the Syrian military and its ally Russia conducted deadly air raids on areas under Islamist and Turkish-backed rebel control.

Assad is backed by Russia and Iran, both of which have confirmed they will help his army fight back after Aleppo, the country's second city, fell out of government control.

Syria has been at war since Assad cracked down on democracy protests in 2011, and the conflict has since drawn in foreign powers and jihadists, and left 500,000 people dead.

With Assad's army back in control of much of the country after years of shifting battle lines, the conflict had been mostly dormant until last week, when the Islamist-led rebel alliance launched the offensive.

"The terrorist escalation reflects the far-reaching goals of dividing the region and fragmenting the countries in it and redraw the map in line with the objectives of the United States and the West," Assad said in a statement from his office.

During the weekend, Islamist Hayat Tahrir al-Sham (HTS) group and allied factions took over Aleppo city, barring neighbourhoods controlled by Kurdish forces, according to the Syrian Observatory for Human Rights.

Aleppo, an ancient city dominated by its landmark citadel, is home to two million people, and saw fierce battles earlier in the conflict.

But until this weekend, the rebels were never able to totally take over.

AFPTV footage showed rebels in military fatigues patrolling Aleppo's streets, some setting fire to a Syrian flag and others holding up the green, red, black and white flag of the revolution.

While the streets appeared to be mostly empty, some were out to cheer for the advancing fighters.

HTS is an alliance led by Al-Qaeda's former Syria branch. It is fighting alongside allied factions, with units taking orders from a joint command.

On Monday, air raids conducted jointly by Syria and Russia on several areas of Idlib province in northwest Syria killed 11 civilians, including five children, said the Observatory.

"The strikes targeted... displaced families living on the edge of a displacement camp," said Hussein Ahmed Khudur, a 45-year-old teacher who sought refuge at the camp from fighting in Aleppo province.

He said one of the children who died was a student of his, and that the other four were his sisters.

Russia, which first intervened directly in the Syrian war in 2015, said Monday it continued to support Assad.

"We of course continue to support Bashar al-Assad and we continue contacts at the appropriate levels, we are analysing the situation," said Kremlin spokesman Dmitry Peskov.

Iran's top diplomat, Abbas Araghchi, was in Syria on Sunday to deliver a message of support, state media said.

On Monday, Iranian foreign ministry spokesman Esmail Baqaei said the Islamic republic had entered Syria at the official invitation of Assad's government.

"Our military advisers were present in Syria, and they are still present. The presence of advisers from the Islamic Republic of Iran in Syria is not a new thing," he said.

- 'Limited utility' -

While the fighting is rooted in a war that began more than a decade ago, much has changed since then.

Millions of Syrians have been displaced, with about 5.5 million living in neighbouring countries.

Most of those involved in the initial anti-Assad protests are either dead, in jail or living in exile.

Russia is at war in Ukraine, and Iran's militant allies Hezbollah and Hamas have been massively weakened by more than a year of conflict with Israel.

The role of Lebanon's Hezbollah, which played a key role in backing the government particularly around Aleppo, remains in question after it withdrew from several of its positions to focus on fighting Israel.

HTS and its allies began their offensive on Wednesday, just as a ceasefire took effect in Lebanon after more than a year of war between Hezbollah and Israel.

The violence in Syria has killed more than 457 people, mostly combatants but also including at least 72 civilians, according to the Observatory, which has a network of sources inside Syria.

The Observatory said rebel advances met little resistance.

Aaron Stein, president of the US-based Foreign Policy Research Institute, said "Russia's presence has thinned out considerably and quick reaction air strikes have limited utility".

He called the rebel advance "a reminder of how weak the regime is".

- 'Lost' -

Aron Lund of the Century International think tank said: "Aleppo seems to be lost for the regime."

He added: "And a government without Aleppo is not really a functional government of Syria."

Rebels have also taken dozens of towns across the north, including Khan Sheikhun and Maaret al-Numan, roughly halfway between Aleppo and Hama, the Observatory said.

AFPTV footage showed rebels pushing into Hama province in central Syria, as well as army tanks and equipment abandoned on the roadside.

The United States and its allies France, Germany and Britain called Sunday for "de-escalation" in Syria, and for the protection of civilians and infrastructure.

The United States maintains hundreds of troops in northeast Syria as part of an anti-jihadist coalition.

Parallel to the HTS-led offensive, pro-Turkey factions in northern Syria attacked Kurdish fighters in Aleppo province on Sunday, with the Observatory saying they seized the strategic town of Tal Rifaat and nearby villages.

On Monday, in light of the takeover of Tal Rifaat, a US-backed, Kurdish-led force in Syria said it is seeking to evacuate Kurds around Aleppo to safe areas under its control.