Since launching a broad offensive against the jihadists nearly three weeks ago, the army has taken around 250 square kilometres (100 square miles) of territory, it said in a statement carried by official news agency SANA.
It said the area includes more than 32 towns and fields.
"This achievement widens the security perimeter around Aleppo city and forms a launching pad for developing military operations against Daesh," the army said, using an Arabic acronym for IS.
It was the biggest blow to Syria's rebel movement since the uprising against Assad erupted in March 2011.
Syria's army on today also said it had seized a 16 kilometre-stretch (10 miles) of highway linking Aleppo city to the IS-held town of Al-Bab.
Al-Bab, 25 kilometres (15 miles) south of the Turkish border, is the last remaining IS bastion in Aleppo province and is seen as a prize by nearly all sides of Syria's complex war.
Turkish President Recep Tayyip Erdogan pledged last month that "the operation in Syria's Al-Bab will be finished in a short time, God willing."
But Ankara's involvement has angered Damascus, which today said it delivered two letters to the United Nations Security Council rebuking Turkey's military operations in Syria.
"In recent days, Turkish armed forces have carried out an incursion into Syrian territory and have occupied some Syrian villages, including two west of Al-Bab," said the letters, sent by the foreign ministry and published by SANA.
The letters accused Ankara of supporting "terrorism" and said the Security Council should act to "put an end to the violations of the Turkish regime".
In January, Turkey's air force began carrying out joint bombing raids around Al-Bab with Assad ally Russia.
The two parties back opposing sides in the war but have joined forces in recent months to try to bring an end to the bloody conflict.
More than 310,000 people have been killed in Syria and millions have been displaced since the uprising broke out nearly six years ago.
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