The news of Qaim's liberation comes the same day that the Syrian military reportedly took back control of Deir Ezzor, the terror group's last-held city in that country.
Qaim is located in the western part of Anbar province, along the Iraq-Syria border, where ISIS holds the last of its territory - the remnants of its so-called caliphate.
Gaining control of Deir Ezzor was a significant victory for the Syrian government, and followed ISIS' ousting from Raqqa, the de facto capital of the caliphate.
The city is the provincial capital of surrounding Deir Ezzor province, an oil-rich region that sits on the county's eastern border with Iraq.
Syrian troops backed by Russian Federation and Kurdish-led forces supported by the United States are now racing toward Boukamal from opposite sides of the Euphrates River, triggering concerns that a proxy showdown could ensue between the two sides.
Syrian troops and allied fighters backed by Russian air power have been battling inside the eastern city since September, when they broke an IS siege of almost three years on government-held districts.
The UK-based watchdog group said the explosion rocked the Tujar, or Merchants, Street between the Conoco gas field and the Jafra energy plants in the eastern province of Deir al-Zour.
In September, the battle to expel ISIS from the city began.
Video broadcast on state-run media showed the last moments of the ferocious fighting in Deir el-Zour, including shelling by Syrian tanks and plumes of smoke rising over the city's mostly destroyed neighborhoods of Jamiayat and Jabiliyeh before they were liberated.
This story was reported by The Associated Press.