Hundreds of US marines have arrived in Syria armed with heavy artillery in preparation for an assault on Isis’ de-facto capital of Raqqa.

A spokesperson for US Central Command told The Independent a Marine Corps artillery unit would provide “greater agility to enable and expedite our Syrian partnered forces defeat of Isis in Raqqa”.

“The strategy adopted by the Coalition remains the same – we will continue to work by, with and through partnered forces to defeat Isis in Iraq and Syria,” he added.

“Our partners in Syria face an entrenched foe and like the Iraqis, will require additional support.”

Col John Dorrian, a spokesperson for Operation Inherent Resolve, said the artillery unit and additional Army Rangers had arrived in recent days but would not have a frontline role.

“We are talking about an additional 400 or so forces in total, and they will be there for a temporary period,” he told Reuters.

The deployment bolsters around 500 special forces troops who are already on the ground to train and support the Syrian Democratic Forces (SDF), who have driven Isis out of swathes of territory in northern Syria.

With backing from US-led coalition air strikes, the Kurdish-led alliance has been fighting to isolate Raqqa city for months, blocking another main road out of the stronghold this week.

The 11th Marine Expeditionary Unit has arrived in the region armed with 155mm howitzers ready to be positioned for the offensive.

The move mirrors tactics ahead of the assault on Mosul in neighbouring Iraq, where the US set up bases outside the city of use as logistics and heavily artillery hubs.

Progress by Iraqi forces has slowed after in the western half of the city as Isis responds with car bombs, drones, suicide attacks and ambushes after digging tunnels between houses and setting booby traps for advancing troops.

The battle could act as a blueprint for a future assault on Raqqa, where up to 4,000 Isis fighters have been reinforcing the city of 300,000 residents for three years.

The marines’ deployment is believed to have been planned under Barack Obama’s administration, with details of a new plan to defeat Isis yet to be made public after being sent to the White House last month.

In pictures: Mosul offensive Show all 40 1 /40 In pictures: Mosul offensive In pictures: Mosul offensive A doctor carries an Iraqi newborn baby at a hospital in Mosul, Iraq July 18, 2017. Reuters In pictures: Mosul offensive Iraqi girls play at a yard of a school in Mosul, Iraq July 18, 2017alal Reuters In pictures: Mosul offensive A woman on crutches who is a relative of men accused of being Islamic State militants is seen at a camp in Bartella, east of Mosul, Iraq July 15, 2017. Picture taken July 15, 2017. Reuters In pictures: Mosul offensive A displaced girl, who fled from home carries a doll at Hamam al-Alil camp south of Mosul, Iraq July 13, 2017. Reuters In pictures: Mosul offensive Iraqi federal police members and civilians celebrate in the Old City of Mosul on 9 July 2017 after the government's announcement of the "liberation" of the embattled city. Iraqi Prime Minister Haider al-Abadi's office said he was in "liberated" Mosul to congratulate "the heroic fighters and the Iraqi people on the achievement of the major victory" AFP/Getty Images In pictures: Mosul offensive A picture taken on 9 July 2017, shows a general view of the destruction in Mosul's Old City. Iraq will announce imminently a final victory in the nearly nine-month offensive to retake Mosul from jihadists, a US general said Saturday, as celebrations broke out among police forces in the city. AFP In pictures: Mosul offensive Members of the Iraqi federal police raise the victory gesture as they ride on a humvee while advancing through the Old City of Mosul on 28 June 2017, as the offensive continues to retake the last district held by Islamic State (IS) group fighters. AFP/Getty Images In pictures: Mosul offensive Smoke billows as Iraqi forces advance through the Old City of Mosul on 26 June 2017, during the ongoing offensive to retake the last district held by the Islamic State (IS) group. AFP/Getty Images In pictures: Mosul offensive An Iraqi man wearing the green scarf of the Shi'ite faith kisses an Iraqi Army soldier on safely reaching the Iraqi forces position as Iraqi civilians flee the Old City of west Mosul where heavy fighting continues on 23 June 2017. Iraqi forces continue to encounter stiff resistance with improvised explosive devices, car bombs, heavy mortar fire and snipers hampering their advance. Getty Images In pictures: Mosul offensive A picture taken from the inside of an Iraqi forces armoured vehicle shows residents walking through a damaged street as troops advance towards Mosul's Old City on 18 June 2017, during the ongoing offensive to retake the last district still held by the Islamic State (IS) group. Military commanders told AFP the assault had begun at dawn after overnight air strikes by the US-led coalition backing Iraqi forces. They said the jihadists were putting up fierce resistance. AFP/Getty Images In pictures: Mosul offensive Iraqi Army soldiers advance in a destroyed street after an Iraqi forces airstrike targeted an Islamic State sniper position 17 June 2017 in al-Shifa, the last district of west Mosul under Islamic State control. IS snipers, as well as car and suicide bomb attacks continue to hinder the Iraqi forces efforts to retake the final district. A series of airstrikes by Iraqi helicopter gunships attempted to hit multiple Islamic State sniper positions in al-Shifa. Getty Images In pictures: Mosul offensive An Iraqi soldier frisks a displaced Iraqi man at a temporary camp in the compound of the closed Nineveh International Hotel in Mosul on 16 June 2017 which was recovered by Iraqi troops from Islamic State group fighters earlier in the year. A screening centre set up in the compound's fairgrounds sees a constant stream of Iraqis fleeing the battle for Mosul, awaiting their turn to be checked by the Iraqi forces who are searching for suspected Islamic State (IS) group members. The small fairground lies at the end of a pontoon bridge across the Tigris recently opened to civilians that is the only physical link between the two banks of the river. AFP/Getty Images In pictures: Mosul offensive Iraqis staying at the al-Khazir camp swim in a river near the camp for internally displaced people, located between Arbil and Mosul on 11 June 2017. AFP/Getty Images In pictures: Mosul offensive Iraqi government forces drive on a road leading to Tal Afar on 9 June 2017, during ongoing battles to retake the city from Islamic State (IS) group fighters. AFP/Getty Images In pictures: Mosul offensive An Iraqi policeman carries a poster bearing an image of Mosul's iconic leaning minaret, known as the "Hadba" (Hunchback), on 22 June 2017. AFP/Getty Images In pictures: Mosul offensive Iraqis stand in line to receive food aid in western Mosul's Zanjili neighbourhood on 7 June 2017, during ongoing battles as Iraqi forces try to retake the city from Islamic State (IS) group fighters. Living conditions in Mosul have again deteriorated since the start of the Iraqi government's offensive on the city in October in which they retook a large part of the west of the city. AFP/Getty Images In pictures: Mosul offensive Displaced Iraqis carry lightbulbs and sacks as they evacuate from western Mosul's Zanjili neighbourhood as government forces advance in the area during their ongoing battle against Islamic State (IS) group fighters on 13 May 2017 AFP/Getty Images In pictures: Mosul offensive A member of the Iraqi Counter-Terrorism Service (CTS) flashes the victory gesture as he patrols in western Mosul's al-Islah al-Zaraye neighbourhood on 13 May 2017 AFP/Getty Images In pictures: Mosul offensive Iraqi army soldiers from the 9th armoured division on a truck flash the sign of victory as they drive back from Mosul to the town of Qaraqosh (also known as Hamdaniya) Getty In pictures: Mosul offensive Members of Iraqi forces flash the sign of victory on their vehicle as they advance towards Hammam al-Alil area south of Mosul Getty In pictures: Mosul offensive A member of Iraqi security forces gestures in Hammam al-Alil, south of Mosul, Iraq Reuters In pictures: Mosul offensive Iraqi children, one flashing the sign of victory, greet Iraqi army's soldiers from the 9th armoured division in the area of Ali Rash, adjacent to the eastern Al-Intissar neighbourhood of Mosul Getty In pictures: Mosul offensive Peshmerga forces look at a tunnel used by Islamic State militants near the town of Bashiqa, east of Mosul, during an operation to attack Islamic State militants in Mosul, Iraq Reuters In pictures: Mosul offensive An Iraqi soldier takes a photograph with his phone as his comrade stands next to a detained man, whom the Iraqi army soldiers accused of being an Islamic State fighter, who was fleeing with his family in the Intisar disrict of eastern Mosul, Iraq Reuters In pictures: Mosul offensive Iranian Kurdish female members of the Freedom Party of Kurdistan (PAK) hold a position in an area near the town of Bashiqa, some 25 kilometres north east of Mosul Getty In pictures: Mosul offensive Iraqi families, who fled their homes in Hamam al-Alil, gather on the outskirts of their town Getty In pictures: Mosul offensive Displaced people walk past a checkpoint near Qayara, south of Mosul, Iraq AP In pictures: Mosul offensive Iraqi families who were displaced by the ongoing operation by Iraqi forces against jihadists of the Islamic State group to retake the city of Mosul, are seen gathering in an area near Qayyarah In pictures: Mosul offensive A boy who just fled Abu Jarbuah village is seen with his family at a Kurdish Peshmerga position between two front lines near Bashiqa, east of Mosul, Iraq Reuters In pictures: Mosul offensive An Iraqi child eats a pomegranate upon the arrival of Iraqi forces in the village of Umm Mahahir, south of Mosul Getty In pictures: Mosul offensive People who just fled Abu Jarbuah village sit as they eat at a Kurdish Peshmerga position between two front lines near Bashiqa, east of Mosul, Iraq Reuters In pictures: Mosul offensive A couple who just fled Abu Jarbuah village are escorted by Kurdish Peshmerga soldiers Reuters In pictures: Mosul offensive Women carry a boy over a wall as civilians flee their houses in the village of Tob Zawa, Iraq AP In pictures: Mosul offensive An Iraqi soldier and a civilian ride a motorbike as smoke rises behind them, on the road between Qayyarah and Mosul Getty In pictures: Mosul offensive A member of Iraqi forces, wearing a skull mask, waits at a checkpoint for people fleeing the main hub city of Mosul Getty In pictures: Mosul offensive An Iraqi soldier sits at a checkpoint in an area near Qayyarah Getty In pictures: Mosul offensive Iraqi men prepare food portions for Iraqi forces deployed in areas south of Mosul Getty In pictures: Mosul offensive Iraqi forces celebrate upon the arrival of vehicles bringing food to them Getty In pictures: Mosul offensive Iraqi childen smoke cigarettes upon the arrival of Iraqi forces in the village of Umm Mahahir, south of Mosul Getty In pictures: Mosul offensive A member of Iraqi forces distributes drinks to children in the village of Umm Mahahir, south of Mosul Getty

Under existing limits put in place by the Obama administration, the military can have up to 503 troops in Syria but temporary personnel like the latest deployment do not count against the cap.

The US is also preparing to send hundreds of service personnel to Kuwait to be ready to join battles against Isis if needed.

The marines’ arrival risks worsening tensions with Turkey, which considers some groups in the US-backed SDF to be terrorists and is supporting Free Syrian Army factions to push them back from the Turkish border.

Despite being allies in the fight against Isis, skirmishes have broken out between the SDF and rival groups of rebels deployed by Turkey, who want to push Kurds out of the city of Manbij and back east of the Euphrates river.

The Manbij Military Council has handed territory on the frontline over to Syrian government forces in efforts to form a buffer between the two sides, while the US has deployed dozens of army troops in what the Pentagon called a “reassure and deter” mission.

Flying American flags and moving in large armoured vehicles, they have been stationed on the outskirts of Manbij to prevent clashes.

Capt Jeff Davis, a spokesman for the Pentagon, said: “It's a visible reminder, for anybody who's looking to start a fight, that the only fight that should be going on right now is with Isis.”

The converging frontlines near al-Bab between parties in the Syrian civil war. Territory controlled by Isis is seen in black, the Syrian government in red, Turkey-backed opposition in blue/green, Kurdish-led Syrian Democratic forces in yellow and Kurdish territory occupied by government forces in orange (Liveuamap)

Turkey has been angered by the US’ overt support for the SDF, whose main component – the Kurdish People’s Protection Units (YPG) – is accused of links with the “terrorist” PKK.

Binali Yildirim, the Prime Minister, voiced his opposition to the US choosing to cooperate with the YPG in the Raqqa offensive.

“If this operation is carried out in this manner there will be a cost for Turkey-US relations, because the YPG is a terrorist organisation,” he said.

Col Dorrian claimed the US was “open to a role for Turkey in the liberation of Raqqa” and would continue discussions on military and diplomatic levels.

The commander said the effort to isolate Raqqa was ”going very, very well" and could be completed in a few weeks, adding: “Then the decision to move in can be made.”

Talks have been held between Russian, Turkish and American commanders in an effort to avoid a full-blown proxy war in northern Syria but deep-seated ideological differences between multiple factions are expected to spark continued fighting.

Vladimir Putin’s forces are supporting Syrian President Bashar al-Assad, alongside Iranian troops and Lebanon’s Hezbollah militant group.

Turkey called for Assad to step down at the start of the civil war and is supporting opposition groups, but has quietened its anti-regime rhetoric with a growing focus on pushing back Kurdish groups that have set up an autonomous region stretching almost the entire length of the Syrian border.

Britain, the US and EU are also calling for “political transition” but have shifted initial support of rebel groups to specifically anti-Isis operations, focusing on the SDF.

Syrian Democratic Forces fighters hold up their weapons before the offensive on Raqqa (Reuters/Rodi Said)

US-led coalition air strikes have hit targets around Raqqa in recent days, destroying three Isis fighting positions, two tunnels and vehicles on Tuesday.

More than a dozen civilians were reportedly killed in a bombing raid on a village east of the city, during battles between Isis and the SDF.

The UK-based Syrian Observatory for Human Rights said six children were among 14 members of the same family who died in an air strike on Thursday.

Investigations by the US-led coalition have so far confirmed at least 220 civilians killed in air strikes against Isis in Syria and Iraq, although the Airwars monitoring group puts the figure at more than 2,500.

"We have had what I would describe as a pretty relentless air campaign to destroy enemy capabilities and to kill enemy fighters in that area already,” Col Dorrian said.

"That is something that we are going to continue and intensify with this new capability."

Isis and al-Qaeda linked extremist groups have been excluded from peace talks in Geneva, which will open for a fifth round of discussions on 23 March.

Staffan de Mistura, the UN special envoy for Syria, said governance, a new constitution, elections and counter-terrorism would be the main issues on the table, but little progress was expected between warring parties.