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.
On the humanitarian front, the United Nations said it would begin flying desperately needed aid from Damascus to the northeastern city of Qamishli, which has been inaccessible by road for more than two years.
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.
Director Rami Abdel Rahman said clashes and air strikes around Manbij were ongoing late Thursday.
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.
- 230 coalition strikes -
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.
He said six civilians including a child were killed Thursday by a mine as they tried to flee 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.
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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.
The coalition on Thursday said Thursday that the SDF had yet to penetrate the centre of Manbij.
British Major General Doug Chalmers told reporters that SDF forces were in "the outer element of the city rather than the city proper".
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 of around 5,000 fighters.
- Airlift planned -
The Manbij assault has coincided with another offensive launched by Syrian regime forces against IS in its stronghold province of Raqa.
Three Russian soldiers supporting regime troops in the area were seriously wounded on Tuesday when their vehicle hit a landmine, the Observatory said, adding that they were recovered by Russian forces.
However, the foreign ministry in Moscow said all its soldiers were "alive, in good health and in their barracks".
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.
The UN on Thursday said it planned to fly in humanitarian aid to the cut-off city of Qamishli, near the Turkish border.
"We have run out of meaningful means to reach people over land," the UN's humanitarian coordinator for Syria, Yacoub El Hillo, told reporters.
He said the airlift "will bring life-saving assistance to a very large number of people".
Qamishli lies in Hasakeh governorate, which can only be reached by road in Syria by driving through the IS-held provinces of Raqa and Deir Ezzor.
Russia and the United States launched a major effort last year to bring about peace talks between the government 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.
And seven civilians, including two children, died in regime air strikes on rebel-held eastern neighbourhoods, the monitor added.