Warplanes have bombed the Islamic State group's de facto Syrian capital Raqa, killing at least 25 civilians, after the jihadists drove pro-government forces out of their bastion northern province.

Twin offensives aimed at severing the jihadists' supply line from the Turkish border to Raqa city appear to have largely stalled as IS mounts a fierce defence using suicide bombers.

Six children were among the 25 civilians killed in bombing raids on Raqa city Tuesday, said the Syrian Observatory for Human Rights.

"Dozens more were wounded, some of them critically," said the British-based monitor, adding they were likely carried out by regime ally Russia.

The Syrian government, Russia and a US-led coalition have all carried out air strikes against IS in Raqa.

The Observatory said fresh raids, apparently by the coalition, also hit the city Wednesday.

Raqa is Being Slaughtered Silently (RBSS) -- an anti-IS activist group which gathers news on atrocities in the city -- posted photos of what it said were the aftermath of Tuesday's strikes.

They show a concrete balcony hanging off the damaged facade of a residential building as a large fire engulfs a white minivan.

The group has accused IS of preventing civilians from leaving the city in order to use them as human shields.

RBSS activist Abu Mohammad told AFP that Tuesday's wounded were struggling to get proper medical treatment as IS has recruited most doctors in the city to treat its own fighters.

- 'Disastrous' retreat -

Raqa city was seized by IS in early 2014 and regime forces were expelled from the entire province that year.

Backed by Russian warplanes, government forces re-entered the province this month as part of an offensive to retake Tabqa, a key town on IS's supply route from Turkey to Raqa city.

But after advancing to within seven kilometres (four miles) of Tabqa airbase, they were driven back late Monday in a jihadist attack that killed 40 loyalists.

A tribal militant who had fought alongside government forces recounted how the army had first been slowed down by mines planted by IS.

"Then Daesh used a huge number of rockets and other explosives to attack the army," which was forced to withdraw from its main outposts, he told AFP, using the Arabic acronym for IS.

Pro-government website Al-Masdar said the IS offensive had led to a "disastrous turn of events" and "a disorganised retreat that left behind weapons and several soldiers".

Washington-based analyst Fabrice Balanche said the pullback could be attributed to a lack of "elite forces" engaged in the battle.

"At the first suicide attacks, they retreated," he said. "The Syrian forces were spread too thin to be defendable."

Further west in the adjacent province of Aleppo, another assault aimed at blocking IS supplies has stalled.

The US-backed Syrian Democratic Forces encircled the jihadist-held town of Manbij on June 10 but have since faced a barrage of IS suicide attacks.

Abu Ibrahim, an SDF field commander stationed near Manbij, said IS attacked two villages east of the town on Wednesday morning.

"They used a car bomb and tried to break through our lines of defence, but the SDF was able to block the attack," he said.

Coalition warplanes "weren't leaving the sky" and had quickly responded to the SDF's call for help, he added.

- 'Civilians are starving' -

On Wednesday, Syrian President Bashar al-Assad named electricity minister Imad Khamis as the new prime minister and tasked him with forming a cabinet.

The 54-year-old engineer replaces Wael al-Halqi who had held the post since August 2012.

Syria's conflict began in 2011 with the brutal repression of anti-government demonstrations. It has killed more than 280,000 people and displaced millions.

Peace efforts have failed to end the violence, and a truce brokered by the US and Russia has all but collapsed.

UN mediator Staffan de Mistura hopes peace talks can resume in July, but has warned they cannot proceed "while hostilities are escalating and civilians are starving"

Negotiations could restart if the truce was reinforced, humanitarian aid was increased, and a "common understanding of a political transition" was reached, he said Tuesday.

"Then we can have, hopefully in July, inter-Syrian talks that are not about principles but about concrete steps to a political transition."

The main Syrian opposition body -- the High Negotiations Committee -- has called for a transitional government body without Assad.

But Syria's regime says Assad is a "red line" and it would only be willing to broaden the government structure to accommodate some opposition figures.