Syrian army and allied forces have taken Albu Kamal, the last major town held by so-called Islamic State in the country, pro-government sources say.
Iraqi Popular Mobilisation Forces also took part in the operation, close to the border between the two countries.
The two groups had been making significant gains against IS in their respective countries in recent weeks.
The militants overran vast areas of both Syria and Iraq in the summer of 2014, establishing a “caliphate”.
A commander in the Syrian pro-government military alliance said, quoted by Reuters: “The last stronghold of Daesh (Islamic State], Albu Kamal, is free of the Daesh organisation.”
The commander said hundreds of members of the Iran-backed Lebanese Shia Hezbollah militia had taken part in the fighting.
Syrian state media have not yet confirmed the capture of Albu Kamal. Sana news agency said Syrian and Iraqi forces had met on the border and the town had been surrounded.
Syrian forces captured the provincial capital, Deir al-Zour, from IS on Friday. Later the same day, Iraqi forces took al-Qaim, across the border.
The US-backed Syrian Democratic Forces (SDF) have also been carrying out offensives in the area. Last month they captured IS’s self-styled capital, Raqqa.
IS retains control over some areas of desert and villages nearby, and a few other pockets in both Iraq and Syria.