Some 182 civilians have been killed since US-backed, Kurdish-led forces launched an offensive to seize the strategic northern Syrian city of Minbij from Islamic State jihadists at the end of May, a monitoring group said on Sunday.

The victims included 56 children, the Syrian Observatory for Human Rights said. Some 64 of them, including 22 children, died in airstrikes by a US-led coalition.

The Kurdish-led Democratic Forces of Syria (DFS) launched an offensive against Minbij, which lies about 30 kilometres from the Turkish border, on May 31 backed by intensive coalition airstrikes.

Capturing the city and its hinterland would cut Islamic State off from its last access route to the border and the outside world.

The DFS quickly surrounded Minbij but have made slow progress since then, with Islamic State jihadists fiercely resisting in street fighting inside the city.

According to analysis firm IHS, in a report also released on Sunday, the fall of Minbij would have a major impact on Islamic State's ability to bring supplies and fighters to its de facto Syrian capital of al-Raqqa and its remaining territories in Syria and Iraq.

Meanwhile, the Observatory said that 29 rebel and al-Qaeda fighters, including field commanders, had been killed in fierce battles as they attempted to force government troops back from the last road into rebel-held eastern Aleppo.

Government forces cut off the Castello Road on Thursday, effectively imposing a siege on the estimated 250,000 to 300,000 residents remaining in the divided city's bomb-shattered eastern sector.

Also on Sunday, the World Food Programme announced that it had airlifted 40 tons of food to the mostly Kurdish-held north-east of Syria.

A total of 25 flights are planned over the coming month, prioritizing supplies for displaced people in shelters and unfinished buildings, female-headed households and some vulnerable Iraqi refugees, the UN agency said.

Syria's north-eastern al-Hassakeh province has been inaccessible for aid agencies as land routes within the country are controlled by Islamic State and the Turkish and Iraqi borders have been largely shut in recent months.

Aid deliveries have been a contentious issue in the Syrian war, with the government frequently preventing or restricting convoys to rebel-held areas besieged by its forces despite widespread malnutrition and some cases of death by starvation.

Three besieged areas on the outskirts of Damascus have received aid supplies, for the first time since 2012, after an international deadline for relief provisions expired at the beginning of June.

The United States, Russia - an ally of the Syrian government - and other countries had jointly announced that if humanitarian aid was denied to besieged areas from June 1, WFP would pursue air drops.

The UN, however, subsequently made it clear that it was still focussing on getting approval for overland deliveries, saying airlifts would be technically difficult and could not be carried out without government approval.

The Syrian conflict, which began with peaceful protests against al-Assad's regime, has turned into a four-way war between the government, mainly Islamist rebels, the Islamic State extremist group and leftist Kurdish forces.

Some 4.8 million Syrians have fled the country since 2011, with another 6.6 million displaced inside its borders, according to the latest UN estimates.

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