Syrian and allied forces converged on Saturday on regrouping ISIL fighters in the Syrian border town of Albu Kamal, the militants' very last urban bastion following a string of losses.
On Friday, Russian-backed Syrian regime forces took full control of Deir Ezzor, the last city where ISIL still had a presence after being expelled from Hawija and Raqqa last month.
Over the border in Iraq, meanwhile, pro-government forces retook Al Qaim, further reducing the militants' hold on territory.
The town lies along the Euphrates River in western Iraq and faces Albu Kamal, which is where many of ISIL's remaining fighters are thought to have regrouped.
The Syrian army and allied militia groups were still some 30 kilometres from Albu Kamal on Saturday evening, but Iraqi paramilitaries had already crossed the border to take on ISIL, the Syrian Observatory for Human Rights said.
"Fighting pitted Hashed Al Shaabi units against the Islamic State in the Hiri area," said Rami Abdel Rahman, who heads the Britain-based monitor.
Hiri, which lies just across the border from Al Qaim, on the outskirts of Albu Kamal, is now the last town of note still fully controlled by ISIL.
Mr Abdel Rahman said ISIL was able to pin back the Iraqi forces.
The Hashed Al Shaabi (Popular Mobilisation Forces) is an umbrella group of mainly Shiite militias loyal to Tehran.
The Syrian regime forces, backed by intensive Russian air strikes, were meanwhile advancing on Albu Kamal from an oil pumping station in the desert west of the town.
Further north, in the east Syria province of Deir Ezzor, US-backed fighters from the Kurdish-led Syrian Democratic Forces alliance were making fresh gains, the Observatory said.
The offensives were more simultaneous than co-ordinated in the border area, where the myriad armed forces involved in the anti-ISIL fight support conflicting agendas.
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The Euphrates Valley border area was the heart of the so-called "caliphate" that ISIL proclaimed in 2014 and is now its last redoubt. The US-led anti-ISIL coalition said around 1,500 of the militants remain there.
The parallel offensives have sent thousands of civilians running for their lives, some of them straight into the desert.
Sonia Khush, Syria director at the Save the Children charity, said an estimated 350,000 people had fled the recent fighting in Deir Ezzor province, half of them children.
"The situation in the city, and surrounding countryside, has been especially bleak with civilians trapped between the fighting and all too often caught in the crossfire," she said.
The Observatory said civilians were stranded on an island in a meander of the Euphrates directly facing Deir Ezzor and where some militant pockets remained.