IS's self-proclaimed caliphate is on the verge of final defeat, with Syrian government forces capturing its last major city on one side of the border and Iraqi forces taking its last substantial town on the other.
The losses on either side of the frontier appear to reduce the caliphate that once ruled over millions of people to a single Syrian border town, a village on a bank of the Euphrates in Iraq and some patches of nearby desert.
Officials on both sides of the border said its final defeat could come swiftly, although they still fear it will reconstitute as a guerilla force, capable of waging attacks without territory to defend.
Iraq's Prime Minister Haidar Abadi announced on Friday that government forces had captured al-Qaim, the border town where the Euphrates spills from Syria into Iraq.
That leaves just the village of Rawa further down river on the opposite bank still in the hands of the ultra-hardline militants, who swept through a third of Iraq in 2014.
On the Syrian side, government forces declared victory in Deir al-Zor, the last major city in the country's eastern desert where the militants still had a presence. Government forces are now about 40km away from Albu Kamal, the Syrian town across the border from al-Qaim, and preparing for a final confrontation.
A US-led international coalition which has been bombing IS and supporting ground allies on both sides of the frontier said before the fall of al-Qaim that the militant group had just a few thousand fighters left, holed up in the two towns on either side of the border.
"We do expect them now to try to flee, but we are cognisant of that and will do all we can to annihilate IS leaders," spokesman US Colonel Ryan Dillon said.
"As IS continues to be hunted into these smallest areas ... we see them fleeing into the desert and hiding there in an attempt to devolve back into an insurgent terrorist group," said Dillon.
"The idea of IS and the virtual caliphate, that will not be defeated in the near term. There is still going to be an IS threat."
The group's leader, Abu Bakr al-Baghdadi, is believed to be hiding in the desert near the frontier.
Driven this year from its two de facto capitals - Iraq's Mosul and Syria's Raqqa - IS has been squeezed into an ever-shrinking pocket of desert straddling the frontier by enemies that include most regional states and global powers.
