Government forces control part of Deir Ezzor city and the adjacent military airport, but IS holds the vast desert province and most of the provincial capital.
"Backed by Iranian, Lebanese and Iraqi fighters, the Syrian army entered Deir Ezzor province from the southeast, near the Iraqi border," said Rami Abdel Rahman, who heads the Syrian Observatory for Human Rights.
He said the pro-government forces had advanced eight kilometres (five miles) into the province.
Syria's desert, known as the "Badiya", extends over 90,000 square kilometres (35,000 square miles) from central Syria to the borders with Iraq and Jordan to the east and southeast.
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Much of the Badiya has been held by IS, but Syria's army has been chipping away at it for months.
In addition to capturing key oil fields and infrastructure, government forces are keen to break IS's stranglehold on regime-held districts of Deir Ezzor city.
Earlier this month, Syrian government forces reached the eastern border with Iraq for the first time since 2015.
By today, according to the Observatory, army troops were in control of an 85-kilometre (53-mile) stretch of the frontier.
The government's advance has created tensions along the border, where US-led coalition forces are using a garrison to train anti-IS fighters.
On June 8, a US warplane shot down a drone after it dropped munitions near At-Tanaf, after other incidents where the coalition fired on pro-regime forces on the ground as they approached the garrison.
More than 320,000 people have been killed in Syria since the conflict began with anti-government protests in March 2011 before turning into a complex war involving regional and international players.
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