During the battle, Hezbollah forces entered Iraq and the Iraqi Popular Mobilisation Forces crossed into Syria to help capture the town, the commander said.
Coalition airstrikes have declined by more than 50% as US -backed forces in Iraq and Syria have largely destroyed the Islamic State's self-proclaimed caliphate and surviving militants have been killed or fled.
IS retains control over some areas of desert and villages nearby, and a few other pockets in both Iraq and Syria.
The Syrian army, alongside Hezbollah and other Shi'ite militias, and backed by Iran and Russian Federation, have seized swathes of central and eastern Syria in the advance against Islamic State this year.
Amid lightning gains that saw the group control almost half of Iraq and Syria at its height in 2014, Baghdadi beckoned Muslims from around the world to join his militant cause during a speech from the Grand Al-Nuri Mosque in Mosul, his first and only known public appearance as ISIS chief.More news: Apple may be working on an AR headset for 2020
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This progress also comes after the Syrian forced and Iraqi troops met at the border, the source added. The extremist group's self-styled caliphate is all but gone, but it is expected to continue inspiring and carrying out attacks.
The victory in al-Bukamal comes hours after the army started storming the city, which was the last IS stronghold in that part of the country near the Iraqi border. Recent reports have revealed that coalition airstrikes have declined by more than 50 percent.
Syrian forces captured the provincial capital, Deir al-Zour, from IS on Friday.
"Our movements are carried out under orders from the commander in chief of the armed forces and our key objective is to liberate Iraq's territories from Daesh".