BEIRUT - US-backed Syrian fighters advancing on the Islamic State group in the strategic northern town of Manbij have progressed to within five kilometres (three miles) of the militant bastion, a monitor said Sunday.

Supported by air strikes by the US-led coalition battling IS in Syria and Iraq, the Syrian Democratic Forces (SDF) alliance of Kurdish and Arab militias launched an assault last week on Manbij. "The Syrian Democratic Forces are now within about five kilometres of the strategic city of Manbij," said the Syrian Observatory for Human Rights, a Britain-based monitoring group.

Manbij is located along a route connecting Raqa - the IS group's de facto capital in Syria - to the Turkish border, a vital conduit for supplies and foreign fighters.

At least 74 people have died in the fighting since the start of the offensive last Monday, including 32 civilians mainly killed as a result of coalition air strikes, said the Observatory, which relies on a network of medics and activists to monitor the conflict.

Thirty militants were also killed, it said, along with 12 SDF fighters.

After taking the village of Khirbet al-Rus, about 20 kilometres (12 miles) southeast of Manbij, the SDF rescued a group of Yazidis - six women and 16 children - who were being held captive by IS, the Observatory said.

The women and children were among hundreds of Yazidis taken captive in mid-2014 as IS carried out a brutal campaign of massacres, enslavement and rape against members of the Yazidi minority.

US Central Command spokesman Colonel Patrick Ryder said on Saturday that SDF fighters had seized more than 100 square kilometres (40 square miles) of territory during the advance. More than 55 air strikes have been carried out since the offensive began, he said, adding that the goal was to hamper IS's ability "to move fighters, weapons, finances (and) supplies into and out of Syria and Iraq."

Some 3,000 Arab fighters were taking part in the offensive, backed by around 500 Kurdish militia members, he said, adding that US special forces were working "at the command and control level" in the operation.

Russian-backed Syrian troops are also advancing against IS in Raqa and on Saturday pushed into the province from the southwest, moving to within 40 kilometres (25 miles) of the Euphrates Valley town of Tabqa, site of the country's biggest dam.

Nearly 50 strikes hit rebel-held areas in and around the Syrian city of Aleppo on Sunday in some of the heaviest raids recently by Russian and Syrian government aircraft, residents and a monitor said.

For their part, rebels also hit government-held areas of Aleppo in what Syrian media said was an escalation in mortar attacks on the western districts of what was the country's largest city before the war.

State media said missiles fired on Hamadaniyah, Midan and other neighbourhoods by insurgents left at least twenty dead, mostly civilians, in the second day of intense shelling of government-held areas, which had left t least 24 dead on Saturday.

In the rebel-held eastern sector of the city, the Syrian Observatory for Human Rights said dozens of barrel bombs - oil drums or cylinders packed with explosives and shrapnel- were dropped by military helicopters on several densely populated districts.

A civil defence worker said at least 32 people were killed in the rebel-held parts of the city, with eighteen bodies alone pulled out of the rubble of flattened buildings in the Qatrji neighbourhood, the worst hit.

"This week-long campaign of bombing is very intense and day by day it's getting worse .. it is the worst we have seen in a while," said Bebars Mishal, a civil defence official in rebel-held Aleppo.

The aerial raids on Sunday came in the wake of Friday's strikes on civilian areas that residents said were the most intense in over a month.

The city, which has been divided for years between rebel and government-held zones, has seen many deadly bombardments that have all but destroyed a February ceasefire agreement.

Full control of Aleppo city would be a huge prize for President Bashar al-Assad. Russia's military intervention in September has helped to bolster Assad's government.

The monitor also said the Syrian government raids had targeted the main Castello road that leads into rebel-held Aleppo as part of a campaign to cut the main rebel route in and complete the encirclement of the city's insurgent-held areas.

A Russian defence ministry statement on Sunday accused militant Syrian Islamist groups of firing mortars on the mainly Kurdish-populated Sheikh Maqsood neighourbood in Aleppo that overlooks the Castello road.

The monitor said 13 people, including six children, were killed on Saturday in the Kurdish-run area by insurgent mortars.

Rebels accuse the powerful Kurdish YPG of working hand in hand with the Syrian army to cut the main artery by intensifying their ground attacks on the highway and targeting civilians who use it while Syrian jets pound the route from the air.

The Russians had on Saturday accused militants from radical Islamic groups of bringing at least 1,000 fighters into an area in the southern Aleppo countryside.

The militants have consolidated gains since Friday in the area around the strategic town of Khan Touman, rebels say.

The Nusra Front spearheaded an attack on Khan Touman last month, delivering one of the biggest battlefield setbacks yet to a coalition of foreign Shia fighters, including Iranians and the Lebanese Hezbollah fighting in support of Syrian government forces..

Rebels say Russian jets on Sunday pounded insurgent positions in the area to prevent them from advancing towards the nearby town of Hader, which rebels say is a stronghold of Iranian-backed militias.