Islamic State has been pushed back from its last strongholds in Iraq and Syria as government forces advance against the militant group.
Syrian troops and their allies announced they had taken full control of the eastern city of Deir al-Zour, the capital of a province of the same name.
Across the nearby border with Iraq, troops began entering areas of al-Qaim town.
The radical group has lost more than 96 percent of the territory it once controlled in both countries, according to the US-led coalition.
"The Syrian Arab Army, in co-operation with the supporting forces, has fully liberated Deir al-Zour city," the state-run SANA news agency reported.
"The Syrian army restored security and stability to Deir al-Zour city."
Deir al-Zour has been almost entirely controlled by Islamic State since 2014.
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The extremist militia still controls less than 40 percent of the province, according to the Syrian Observatory for Human Rights.
The army, backed by Russian bombers, Iran and Shi'ite militias, is advancing towards the last significant town held by Islamic State in Syria, Albu Kamal, which is also on the western bank of the Euphrates.
On the other side of the border, Iraqi forces on Friday began entering several neighbourhoods in al-Qaim town as part of their campaign to remove the last traces of Islamic State from the country.
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Iraqi forces, supported by tribal fighters and pro-government Shi'ite militias, stormed the Sada, Karabala and Hasiba areas inside al-Qaim, chief of the western Anbar operations, Lieutenant General Abdul-Amir Yarallah, said.
Prime Minister Haider al-Abadi announced the beginning of the offensive last week, mainly targeting the western town of al-Qaim and Rawa.
Government forces, backed by US-led air power, recaptured the northern city of Mosul, Iraq's second-largest city, from Islamic State in July.
Mosul was their main stronghold in Iraq.