Iraqi forces have launched an offensive to recapture Rawa, the last town controlled by so-called Islamic State.
The town, and a few small parcels of land, are all that remains under the militants' control in Iraq, after the larger town of al-Qaim fell last week.
The group's last urban stronghold in Syria, Albu Kamal, fell afterwards.
Retaking Rawa in the offensive, which began on Saturday, would leave the jihadist group on the verge of complete defeat in Iraq.
The Joint Operations Command said in a statement that two Iraqi infantry divisions and Sunni tribal forces were being supported by "war planes".
Rawa was one of a number of towns on both sides of the Iraqi-Syrian border which IS had designated part of its "Euphrates Province", which it had used to transfer fighters, weapons and goods between the two countries.
The cross-border province was also a symbol of the jihadists' intention to eradicate all of the region's frontiers and lay to rest the 1916 Sykes-Picot agreement, an emblem of the colonial division of the area resented by many Arabs and Kurds.
Albu Kamal and al-Qaim also formed part of this self-styled province.
IS has now been driven out of about 95% of the land the group once held in Iraq and more than 4.4 million Iraqis have been freed from its rule, according to the US-led coalition.
Media captionUS-backed forces may be celebrating, but IS has not been defeated
The group has just "months [remaining] at most as a proto-state", the senior Royal Air Force (RAF) officer overseeing British air strikes against IS in Iraq and Syria said last week.
But Commodore Johnny Stringer told journalists in London that IS would "almost certainly morph into an insurgent organisation" that would try to launch attacks in the two countries.