A war monitor said the army and its Iran-backed allies "suddenly" pulled out of Deir Ezzor province as a rebel offensive dealt President Bashar al-Assad's government a series of stunning blows in northern and central Syria.

"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.

Rami Abdel Rahman, who heads the Syrian Observatory for Human Rights, said "Syrian regime forces and commanders of Iran-backed allied groups suddenly withdrew from Deir Ezzor city and its countryside".

"Columns of soldiers headed towards the central Palmyra region," he told AFP.

Their redeployment towards the oasis town of Palmyra came as the Islamist-led rebels reached the gates of Syria's third city Homs to its west.

Since the collapse of the caliphate proclaimed by jihadists of the Islamic State group in 2014, Deir Ezzor province had been broadly divided between government forces on the right bank of the Euphrates and Kurdish-led forces on the left.

The oil-producing province borders Iraq and had been a key point of entry for Iranian-backed groups fighting alongside Assad's forces.

- Unrest in south -

Assad's troops also relaxed their grip on the south, allowing armed groups in Daraa province, a rebel bastion at the height of the civil war in the early 2010s, to take control of the Nassib-Jaber border crossing with Jordan, the Observatory said.

Jordan closed its side of the crossing, Interior Minister Mazen al-Faraya said.

"Armed factions seized control of the Nassib border crossing with Jordan, as well as nearby checkpoints and towns," the Observatory chief told AFP.

Elsewhere in the province, they captured a police station and an air force intelligence base, Abdel Rahman added.

Daraa province was the cradle of the 2011 uprising against Assad's rule but it returned to government control in 2018 under a ceasefire deal brokered by Assad ally Russia.

Former rebels there who accepted the 2018 deal were able to keep their light weapons.

In Nawa, north of Daraa city, the Observatory said "local fighters managed to control several positions... after a broad attack targeting the military intelligence department".

"In retaliation, regime forces... shelled residential areas in Nawa," the monitor said, adding that the shelling extended to other towns.

Daraa province has been plagued by unrest in recent years, with frequent attacks, armed clashes and assassinations, some claimed by the Islamic State group.

Activists in the province shared images on social media of people chanting anti-Assad slogans in the town of Bosra al-Sham, and waving the rebel flag at the Omari Mosque in Daraa.