Islamic State (ISIS/ISIL) jihadists repelled a Russian-backed Syrian regime offensive in the northern province of Raqqa, killing at least 40 troops loyal to dictator Bashar al-Assad, reports a monitoring group.

The U.K.-based Syrian Observatory for Human Rights, which uses a network of ground sources to monitor the Syrian conflict, notes that ISIS lost 21 of its fighters in the lightning counter-offensive launched Sunday.

Syrian troops, backed by Russian airstrikes, had been trying to retake the ISIS stronghold of Raqqa province since June 3 and were able to advance an estimated 12 miles toward the town of Tabqa.

The province is home to ISIS’s de facto capital of Raqqa city, which is also the provincial capital.

Overall, an estimated 93 Assad troops and 126 ISIS jihadists died throughout the Raqqa offensive. Those figures include the recent fatalities.

“Daesh [ISIS] has managed to drive out regime troops from the administrative borders of Raqqa province after a fierce counter-offensive,” Agence France-Presse (AFP) quotes the observatory as saying.

The monitor group noted that the jihadist group had sent reinforcements from their de facto capital city to defend the town of Tabqa, home to a dam and an air base.

ISIS deployed vehicles and “300 militants” armed “with heavy machine guns,” reports the U.K.-based group.

The regime offensive into Raqqa marked the first time the Syrian troops were able to push into the province since 2014.

AFP notes that the Assad military was “aiming for the country’s largest dam at Tabqa on the Euphrates River.”

On Monday, U.S. Secretary of State John Kerry acknowledged that he had read the memo from frustrated diplomats criticizing the Obama administration’s Syria policy and urging strikes against the Assad regime.

Asked whether he had read the dissenting memo, which was signed by 51 diplomats, Kerry responded, “Yes. It’s very good. I’m going to meet with them.”

The “dissent cable” became public last week when it was leaked to news outlets, including the New York Times (NYT). 

“The memo, filed in the State Department’s ‘dissent channel,’ underscores the deep rifts and lingering frustration within the administration over how to deal with a war that has killed more than 400,000 people,” notes the Times.

“The State Department set up the channel during the Vietnam War as a way for employees who had disagreements with policies to register their protest with the secretary of state and other top officials, without fear of reprisal,” it adds. “While dissent cables are not that unusual, the number of signatures on this document, 51, is extremely large, if not unprecedented.”

Although Kerry has maintained his support of Obama’s Syria strategy through the ongoing five-year-old massacre in Syria, he is reportedly frustrated.

Kerry has been “pushing an improbable joint US-Russian plan to lure Assad and the rebels to the negotiating table,” notes AFP.

The memo calls for “a judicious use of stand-off and air weapons” against the Assad regime to encourage the dictator to seek a negotiated solution.

“Such a step would represent a radical shift in the administration’s approach to the civil war in Syria, and there is little evidence that President Obama has plans to change course,” reports the Times.

The Russian-backed Assad regime has killed and injured more civilians than Islamic terrorists, the observatory has found.