Kurdish-Arab forces launched a major assault against the Islamic State group in Syria's Raqa province on Tuesday and Iraqi forces advanced on it in Fallujah, piling pressure on the jihadists in two strongholds.

The twin offensives marked some of the most serious ground efforts against IS since the group declared its self-styled "caliphate" straddling the Syrian-Iraqi border in 2014.

Territory under IS control has been steadily shrinking for months but it has carried out a wave of attacks including bombings in the Syrian government's coastal heartland on Monday that killed 177 people.

It was the "deadliest bomb attack" on any Damascus-held area in Syria's five-year war.

The US-backed Syrian Democratic Forces on Tuesday announced its largest offensive to date against IS territory north of the jihadists' stronghold of Raqa city.

The offensive was aimed at pushing IS from the province's north and securing other areas, the alliance said in statement on Twitter.

Baghdad-based US military spokesman Colonel Steve Warren confirmed the assault, saying it was "putting pressure on Raqa".

He said the US military would conduct airstrikes to support thousands of SDF fighters, some of whom have been trained and equipped by US forces.

If Raqa falls, "it's the beginning of the end of their caliphate", Warren said.

'End to IS myth?'

Just before the SDF announcement, Russia said it would be ready to coordinate with both Washington and the SDF in an offensive for Raqa.

Meanwhile, three explosions rocked the town of Banyas on Tuesday in Syria's Tartus province, about 400 kilometers west of Raqa city.

Security forces were spread out throughout Banyas but there was no immediate word on casualties or the cause of the blasts.

The US rejected a Russian proposal last week for joint air operations against jihadist groups in Syria.

The anti-IS coalition headed by Washington has set its sights on Raqa in Syria, as well as Fallujah and eventually IS's main bastion of Mosul in Iraq.

"Cutting the route between Raqa and Mosul isn't difficult today. It will put an end to the myth of a transnational IS," said Washington-based Syria analyst Fabrice Balanche. "It's clear that if the US wants to eliminate IS, it has to attack it on multiple fronts at the same time."

Civilians caught

On Tuesday, Iraqi forces closed in on Fallujah after capturing the nearby town of Garma and cutting IS off from one of its last support areas.

With forces converging, concerns grew that the estimated 50,000 civilians believed to still be inside had nowhere to go.

"Families who have been suffering food and medical shortages over the last months now risk being caught in the crossfire," said the Norwegian Refugee Council's country director Nasr Muflahi.