Pressure mounted on the Islamic State group Tuesday as a Kurdish-Arab alliance launched a major assault north of the militants' Syrian bastion of Raqa and Iraqi forces advanced on their stronghold of Fallujah.
The twin offensives marked some of the most serious ground efforts against IS group since the group declared its self-styled "caliphate" straddling the Syrian-Iraqi border in 2014.
Territory under IS group control has been steadily shrinking for months but -- in a sign of its continued ability to mount attacks -- the group has carried out a wave of violence including bombings in the Syrian regime's coastal heartland Monday that killed more than 160 people.
On Tuesday, Kurdish and Arab fighters announced their largest offensive to date against IS group territory north of Raqa, the group's de facto Syrian capital.
The Syrian Democratic Forces -- a seven-month-old alliance between Kurdish and Arab forces -- said it would push IS group from the province's north and secure other areas already seized from the militants.
Its statement, posted on Twitter, made no mention of the provincial capital, Raqa city.
Baghdad-based US military spokesman Colonel Steve Warren confirmed the assault, saying: "The SDF have begun operations to clear the northern countryside, so this is putting pressure on Raqa."
The US military will conduct air strikes in support of the "several thousand" SDF fighters, some of whom have been trained and equipped by the United States, he said.
If Raqa falls, "it's the beginning of the end of their caliphate," Warren said.
Just before the SDF announcement, Russia said it would be ready to coordinate with both Washington and the SDF in an offensive for Raqa.
The US rejected a Russian proposal last week to carry out joint air operations against militants 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.
"It's clear that if the US wants to eliminate IS group, it has to attack it on multiple fronts at the same time," said Washington-based Syria analyst Fabrice Balanche.
"Cutting the route between Raqa and Mosul isn't difficult today. It will put an end to the myth of a transnational IS," he told AFP.
On Tuesday, Iraqi forces closed in on Fallujah after capturing the nearby town of Garma and cutting IS group off from one of its last support areas.
"Federal forces advanced towards the east of Fallujah early today from three directions," police Lieutenant General Raed Shakir Jawdat told AFP.
The Hashed al-Shaabi umbrella paramilitary organisation, dominated by Tehran-backed Shiite militias that are heavily involved in the operation, said ground was also gained south of Fallujah.
With forces converging on the city, 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.
He said it was "absolutely vital that they are granted safe routes out of there so that we can assist them."
Officials from Anbar, the vast western province in which Fallujah is located, reported that small numbers of civilians had managed to sneak out.
A Fallujah resident reached by telephone told AFP there was heavy shelling on the northern edge of the city Tuesday.
"Daesh (IS) is still imposing a curfew, preventing people from coming out on the street," said the man, who gave his name as Abu Mohammed al-Dulaimi.
"The number of Daesh members is decreasing and we have started seeing them walk in the street in groups of two or three. We don't know where the others are," he said.
It was unclear what kind of defence IS group was prepared to put up in Fallujah, a city that looms large in modern jihadist mythology since 2004 battles that saw US forces suffer some of their worst losses since the Vietnam War.
Iraqi and US-led coalition aircraft have been pounding Fallujah and its surroundings in recent days to support the operation.
The offensives came as Washington and Moscow scrambled to salvage a shaky ceasefire between the regime and non-militants rebels intended to pave the way for peace talks to end Syria's five-year conflict.
The US envoy for Syria late Monday urged rebels to respect the February 27 ceasefire after they gave its brokers -- Washington and Moscow -- until Tuesday afternoon to stop an advance on rebel bastions outside Damascus.
"We recognise that the CoH (Cessation of Hostilities) is under severe stress, but believe that to abandon it now would be strategic error," Michael Ratney said in a statement on Twitter.
Staunch regime ally Russia late Monday also called for a 72-hour local truce in Eastern Ghouta and Daraya near Damascus -- within the wider nationwide ceasefire -- from Tuesday.