US-backed Kurdish and Arab fighters advanced Thursday into the Islamic State jihadist group's bastion of Manbij in northern Syria, sparking fierce street fighting as they push to take the city.
Backed by air strikes by the US-led coalition bombing IS in Syria and Iraq, fighters with the Syrian Democratic Forces (SDF) alliance entered Manbij from the south, a monitoring group said.
The advance marked a major breakthrough in the battle for Manbij, once a key link on the supply route between the Turkish border and IS's de facto Syrian capital of Raqa.
The loss of the city would deal another blow to IS following a string of recent battlefield defeats, including the taking by Iraqi forces earlier this month of the centre of the Iraqi city of Fallujah.
The Syrian Observatory for Human Rights, a Britain-based monitor, said SDF forces were able to break through IS defences in Manbij a few hours after taking control of a village on the city's southwestern outskirts.
"Fierce street fighting between buildings" erupted as they entered the city, said Observatory chief Rami Abdel Rahman, whose group relies on a broad network of sources inside Syria to monitor the country's conflict.
An SDF commander at the front told AFP that IS fighters were using car bombs and other explosives to try to slow the assault.
"Our forces, in coordination with the coalition, are determined to advance inside the city and eliminate all Daesh fighters," he said, using an Arabic acronym for IS.
Abdel Rahman said tens of thousands of civilians were trapped inside the city, though some 8,000 had been able to flee since the start of the SDF offensive on Manbij on May 31.
There were fears the jihadists would use civilians as human shields inside the city, which had a population of about 120,000 before the start of Syria's civil war in 2011.
The SDF managed to encircle the city on June 10 but its advance slowed as IS fought back, including with almost daily suicide bombings.
At least 63 SDF fighters and 458 jihadists have been killed since the start of the offensive, according to the Observatory.
The jihadists have held Manbij since 2014, the year IS seized control of large parts of Syria and neighbouring Iraq and declared its "caliphate".
The US-led coalition of Western and Arab states launched air raids against IS in both countries the same year and in recent months has stepped up support for ground forces like the SDF.
A statement from US Central Command said the coalition had carried out 73 strikes in the Manbij area last week and a total of 233 since the assault began.
Formed in October 2015, the 25,000-strong SDF is dominated by the powerful Kurdish People's Protection Units (YPG) but includes an Arab contingent that has been steadily growing to around 5,000 fighters.
As well as air support, coalition countries have provided ground advisors to the SDF, including some 200 US special forces.
The Manbij assault has coincided with another offensive launched by Syrian regime forces against IS in its stronghold province of Raqa.
Backed by Russian warplanes, government forces re-entered the province this month as part of an offensive to retake Tabqa, another key town on the IS supply route to the Turkish border.
But after advancing to within seven kilometres (four miles) of Tabqa airbase, they were driven back late Monday in a jihadist counter-attack that killed 40 loyalists.
Three Russian soldiers supporting regime troops in the area were seriously wounded on Tuesday when their vehicle hit a landmine, the Observatory said. They were recovered by Russian forces.
Syria's conflict began five years ago with the brutal repression of anti-government demonstrations. It has killed more than 280,000 people and displaced millions.
IS emerged from the chaos of the war, committing widespread atrocities in areas under its control, as well as organising and inspiring jihadist attacks across the Middle East and in Western cities.
Washington has backed rebel forces in Syria and Moscow is supporting President Bashar al-Assad's regime, but the rise of IS has seen efforts focus on defeating the jihadists.
Russia and the United States launched a major effort last year to bring about peace talks between Assad and rebel forces, but the negotiations faltered and a partial truce announced in February has all but collapsed.
Clashes have been especially intense in and around Syria's second city of Aleppo, where the Observatory said six people including a child died Thursday in rebel shelling of pro-regime neighbourhoods.