MOSUL • United States-backed Iraqi forces pushed deeper into western Mosul yesterday, advancing in several populated southern districts after punching through the defences of Islamic State in Iraq and Syria's (ISIS') last major urban stronghold in Iraq a day earlier.

About 1,000 civilians also walked across the front line, the largest movement since the new offensive launched last week to deal the ultra-hardline Sunni Muslim group a decisive blow.

The new push in Mosul comes after government forces and their allies finished clearing ISIS from the east of the northern Iraqi city last month, confining the insurgents to the western sector on the other side of the Tigris river.

Commanders expect the battle in western Mosul to be more difficult, in part because tanks and armoured vehicles cannot pass through the narrow alleyways that criss-cross ancient districts there.

The advancing forces are less than 3km from the mosque in the old city, where ISIS leader Abu Bakr al-Baghdadi declared a caliphate spanning Iraq and Syria in 2014, sparking an international military campaign to defeat the group.

Meanwhile, Iraqi jets have struck ISIS targets inside neighbouring Syria for the first time, Iraq's Prime Minister said, vowing to chase down militants "everywhere" as his troops opened new fronts in their battle for the city of Mosul.

The air strikes in Bukamal, Syria, on Friday, were coordinated with Damascus and carried out through a joint intelligence-sharing and command centre in Baghdad involving Syria, Iran, Iraq and Russia, according to Brigadier-General Tahseen Ibrahim, a spokesman for Iraq's Defence Ministry.

But even as it loses ground, ISIS has continued to bomb civilian targets elsewhere in Iraq and Syria, where blasts near the northern town of al-Bab killed at least 50 people on Friday.

"We are determined to follow the terrorism that is trying to kill our sons and our citizens everywhere," Iraqi Prime Minister Haider al-Abadi said.

Iraqi officials said they had information that recent attacks in Baghdad had been organised from inside Syria.

The cars used in those attacks, including one on a market in Baghdad's south-western Bayaa neighbourhood this month that killed at least 45 people, had been rigged with explosives in Bukamal and Husaybah, on the Iraqi side of the border, Iraqi officials said.

Husaybah was also the target of air strikes on Friday.

Iraq decided to carry out the strikes itself, rather than rely on the US-led coalition because it was acting on the basis of Iraqi intelligence and "it's up to us to take revenge", said Lieutenant-General Anwar Hana, commander of Iraq's air force. The strikes were successful, he said.

ISIS has lost over half of the territory it once controlled in Iraq, but headway against the group has been slower in Syria.

But Turkey-backed Free Syrian Army rebels recaptured the town of al-Bab on Thursday, pushing ISIS from its final foothold along Syria's northern border with Turkey.

The offensive, which began in early December last year, has reduced much of al-Bab to a ghost town, its pre-war population of about 100,000 having dwindled to the low thousands.

Suspected ISIS car bombs killed at least 50 people in the village of Sousyan, about 10km north-west of al-Bab, on Friday, witnesses and the opposition activist-run Aleppo Media Centre said.

The attacks hinted at the scope of the challenges facing the Turkey- backed forces as they seek to restore security to areas they have retaken from ISIS.

REUTERS, WASHINGTON POST