With a sudden lunge through extremist lines, the Syrian military and its allies on Monday came to within three kilometres of relieving the Euphrates city of Deir Al Zor, where IS has besieged an army garrison and 93,000 civilians for years.
The advance on the eastern city marks another stinging setback for the once-triumphant IS, fast retreating in both Iraq and Syria as its self-declared caliphate crumbles.
Syrian troops were rapidly approaching the city on Monday, reaching a point three km (2 miles) from their comrades in the city, state television reported.
Deir Al Zors provincial governor said on Sunday he expected the army to reach the city by Tuesday night.
A military media unit run by the governments ally Hezbollah said the advancing forces were heading to the besieged garrisons camp on the city outskirts.
"IS is in confusion. There is no leadership or centralised control," said a commander in the military alliance supporting Syrian President Bashar Al Assad.
Hemmed in on all sides, IS is falling back on a last Euphrates stronghold downstream of Deir Al Zor in the towns of Al Mayadin and Al Bukamal near the border with Iraq.
But as it has lost its core territory - defeated in Iraqs Mosul and yielding street after street in its de facto Syrian capital of Raqqa - the ultra-hardline group has still been able to launch attacks in the West and maintain a threat in other centres such as Libya.
In IS-encircled Deir Al Zor, news of the armys approach prompted people to take to the streets to celebrate, governor Mohammed Ibrahim Samra said by phone.
The city has been cut off since 2013, after rebel groups rose up against Assad during the first flush of Syrias six-year war.
IS then overran rebel positions and surrounded the armys enclave in the city in 2014.
It was a major prize. Deir Al Zor is the centre of Syrias oil industry, a source of wealth to the group and a serious loss to Damascus.
As the army has pushed east in recent months, oil and gas fields have once more fallen to the government.
IS militants have stepped up efforts this year to seize the enclave before the army could arrive.
In January, they managed to sever it from the military airbase in the city and take over a nearby hill, further straining its links to the outside. During the long siege, the city has been supplied via high-altitude air drops.
The United Nations said in August it estimated there were 93,000 civilians in the governments Deir Al Zor pocket, where conditions were "extremely difficult".
"Despite all this and despite the shelling and injured, things are running in the city. The institutions are running, the bakeries. Water is also pumped twice a week to our residents, aid is distributed daily," governor Samra said.
For Assad, the weekends lightning advance caps months of steady progress after government forces turned from their victory over rebels in Aleppo last December to push eastwards against IS.
"The army has been advancing in a rapid and calculated way from all directions," a Syrian military source said, referring to the months-long campaign across the desert.
Aided by Russian jets and an alliance of militias backed by Iran, including Lebanons Hezbollah, the Syrian army has captured swathes of the central and eastern deserts in parallel offensives from Palmyra and Al Resafa.
Those offensives have accelerated since linking up last month, enclosing two big IS pockets and overrunning all but a small portion of one of them, near the town of Al Salamiya.
As the army and its allies have advanced on Deir Al Zor, IS has pulled reinforcements from Al Mayadin and used its usual tactics of booby traps, mines and sudden raids, the commander in the pro-Assad military alliance said.
The very rapid advance in recent days was the result of heavy preparatory bombardment, a multi-pronged assault and gains in high ground commanding the surrounding area, the commander said.
Meanwhile, as the army and its allies have forced other militant pockets to surrender, including an IS enclave on Syrias border with Lebanon a week ago, it has been able to transfer more troops to the desert campaign.
"It helped a lot to switch the military effort of the Syrian army and the resistance to the eastern Syrian desert," the commander said, adding that thousands of troops had arrived from the battle on the Lebanon border.
IS extremists and their families evacuated from that enclave as part of a surrender deal were escorted by the Syrian army and Hezbollah to east Syria, but have been stopped by a U.S.-led coalition from reaching Deir Al Zor.
Ten of the original 17 buses are now stuck in no-mans land between pro-government forces and IS territory and six buses retreated back into government areas, the commander added.