US-backed forces have penetrated the heavily fortified heart of Islamist militant bastion Raqa for the first time, in a key milestone in the war against the Islamic State (IS) militant group in Syria.
Air strikes by the US-led coalition battling IS punched two holes in the mediaeval wall surrounding Raqa's Old City, allowing fighters of the Syrian Democratic Forces to breach the Islamist militant defences, Washington and the SDF said on Tuesday.
The landmark advance in IS's notorious Syria bastion -- the culmination of a nearly eight-month campaign -- comes as the Islamist militants face an expected defeat within days in Iraq's second city Mosul, the other pivot of the cross-border "caliphate" they declared in 2014.
Coalition officials said a few hundred diehard Islamist militants were making a desperate last stand in just one square kilometre (less than half a square mile) of Mosul's Old City.
In neighbouring Syria, US-backed forces -- including the SDF and Arab fighters from the Syrian Elite Forces -- broke into Raqa's Old City.
"There have been fierce clashes (in the Old City) since dawn today, with 200 of our fighters mobilising to the area," Syrian Elite Forces spokesman Mohammad Khaled Shaker told AFP.
The SDF said coalition warplanes opened up two breaches in the 2.5 kilometre (one and a half mile) Rafiqah Wall around the Old City, enabling its fighters to evade explosives laid by IS.
"Daesh (IS) have used this archaeological wall to launch attacks, and planted bombs and mines in its gates to hinder the advance of SDF forces," the alliance said.
Under three years of IS rule, Raqa became infamous as the scene of some of IS's worst atrocities, including public beheadings, and is thought to have been a hub for planning attacks overseas.
The US envoy to the coalition, Brett McGurk, said on Twitter that breaching the Rafiqah Wall marked a "key milestone in campaign to liberate the city."
US Central Command, which oversees military operations across the Middle East, said the coalition air strike had allowed advancing forces "to breach the Old City at locations of their choosing."
This prevented IS from using booby-traps, landmines and suicide car bombs, while it also "protected SDF and civilian lives, and preserved the integrity of the greatest portion of the wall," it said.
The United Nations has raised concerns for tens of thousands of civilians trapped in Raqa, where it says IS are using many as human shields.
The Rafiqah Wall that surrounds the city's historic heart originally dates back to the late 8th Century, when as capital of the Abbasid caliphate, Raqa was briefly the centre of the Islamic world.
The Syrian Observatory for Human Rights monitoring group said the SDF advance had been supported by US special forces and constituted the "most important progress" yet for the SDF.
The alliance broke into Raqa on June 6 and has since chipped at IS positions in the city's west and east, recapturing the eastern district of Al-Senaa on Tuesday.
The US-backed fighters also opened a new front against IS on Sunday, crossing the south bank of the Euphrates River to establish a bridgehead on the north bank.
But IS still controls 70 percent of Raqa city, according to the Observatory, and the toughest battles are yet to come.
"Today, the real fight for Raqa has begun. There are both many civilians and many mines in the Old City," said Observatory chief Rami Abdel Rahman.
"The biggest challenge for the SDF will be opening up humanitarian corridors so besieged civilians can leave," he said.
The coalition estimates that some 2,500 IS are defending Raqa.
That is far more than the 200 or so diehard IS fighters, most of them foreign, that Iraqi commanders believe are holed up in the Old City of Mosul.
Iraqi forces were moving in on the last IS-controlled neighbourhoods of the Old City from all sides on Tuesday, commanders said, adding that they expected to announce victory in as few as two days.
"From the early morning, we were able to gain an important foothold in these neighbourhoods," Staff Brigadier General Haidar al-Obeidi, a commander in the elite Counter-Terrorism Service, told AFP.
"In the next two days, we will announce the complete liberation of the Old City, and therefore... the city of Mosul."
Iraqi forces have been closing in on Mosul's Old City for months, but its maze of narrow alleyways combined with a large civilian population has made for an extremely difficult fight.
Iraqi forces are facing a rising number of suicide attacks, including some by female bombers, in the final stages of the more than eight-month-long campaign, commanders said.
But coalition officials said IS were now on their last legs in their two most emblematic strongholds.
"#ISIS terrorists down to less than one square kilometre in #Mosul and totally surrounded in #Raqqa," tweeted US envoy McGurk.