The territorial ambitions of IS are now all but destroyed - although its ideology remains powerful - after it lost control of two of its last remaining strongholds in Syria and Iraq.
Syria's army announced it had taken Deir al-Zour early on Friday. IS had held most of Deir al-Zour since 2014, important because of its proximity to the border with Iraq.
While the Syrian army said it had been re-captured, other reports said government forces and their allies were clearing the last pockets of resistance from IS in the city.
Later in the day, Iraq Prime Minister Haider al-Abadi said al-Qa'im, near the Syrian border, had been recaptured by government forces.
Al-Qaim was the last sizeable territory held by IS in Iraq. The operation to retake the city and the surrounding area was launched last week. Soldiers, police, Sunni tribesmen and mostly Shia paramilitary fighters, some backed by Iran, took part in the assault.
The town was taken "in record time", Mr al-Abadi said.
Iraqi forces said they had also seized the last border post between Iraq and Syria that was held by IS.
The US-led coalition said IS had now been driven out of about 95 percent of the land the group once held in Iraq, and more than 4.4 million Iraqis had been freed from its rule.
- A US-backed Syrian alliance took control of the former capital of Islamic State's self-styled "caliphate" in Raqqa, Syria last month.
- Mosul - Iraq's second-largest city - was taken from IS in July after months of fighting.
- In Syria, the militant group is now confined to a few pockets in Deir al-Zour province. The coalition says there are about 1500 IS fighters left in the area.
It marks the near destruction of the territorial ambitions of the group, although its ideological influence remained powerful, BBC's Arab Affairs Editor Sebastian Usher said.
The group has just "months [remaining] at most as a proto-state", the senior Royal Air Force (RAF) officer overseeing British airstrikes against IS in Iraq and Syria said on Friday.
However, Commodore Johnny Stringer told journalists in London that IS would "almost certainly morph into an insurgent organisation" trying to launch attacks in the two countries.
The Syrian Democratic Forces (SDF) and Syrian government forces have been carrying out separate offensives in Deir al-Zour, with the aim of taking control of Albu Kamal, a key crossing with Iraq.
In some areas the US-backed SDF and Russia-backed Syrian army have taken up positions just a few kilometres apart.
An SDF spokesman told the BBC they were still encountering some resistance with the militants using suicide cars and trucks, thermal missiles and mortars. In some towns and villages there was house-to-house fighting, Kino Gabriel said.
IS has also suffered a series of defeats in recent months to Iraqi government forces, who were advancing along the Euphrates river on the other side of the border.
IS had designated the area on both sides of the border as its "Euphrates Province" and used it to transfer fighters, weapons and goods between Iraq and Syria.
The cross-border province was also a symbol of the jihadists' intention to eradicate all 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.
Some 350,000 civilians in Syria's Deir al-Zour province have been forced to flee their homes during weeks of fighting.