Syrian government troops and allied fighters, backed by Russian air support, have been advancing for weeks towards Deir Ezzor city.
"The Syrian Arab Army has advanced on the Brigade 137 base front on the western side of Deir Ezzor city and broken the siege imposed by the Daesh organisation," state news agency SANA said, using the Arabic acronym for IS.
Syrian government troops and allied fighters, backed by Russian air support, have been advancing for weeks towards Deir Ezzor city, capital of oil-rich Deir Ezzor province, which borders Iraq.
Government forces and tens of thousands of civilians in the city have been trapped under IS siege for over two years, facing food and medical shortages.
Early this year, the government-held parts of the city were cut in two by an IS offensive.
The army's advance to the Brigade 137 base Tuesday breaks the siege on the northern part of the city, to which it is connected by a road.
But a southern government-held section, including the key military airport, remains surrounded.
Government forces are heading towards the southern sector, however, and are currently around 15 kilometres (nine miles) away, according to the Syrian Observatory for Human Rights, a Britain-based monitor.
Around 100,000 people are believed to be inside government-held areas of Deir Ezzor, with perhaps 10,000 more in parts of the city held by IS.
Earlier Tuesday, a local journalist inside the government-held northern part of Deir Ezzor reported the sound of fierce clashes and heavy artillery as the army approached.
He said the national flag had been raised throughout the area in anticipation of celebrations upon the arrival of government soldiers.
Some residents, who have faced shortages of food and medicine throughout the siege, had begun greeting each other with "Good morning of victory," he added.