The Islamic State militant group is on the rise in Iraq and Syria after crushing Baghdad’s plans to retake Tikrit. Hundreds of troops have been captured and 270 people were murdered in a Syrian gas field following the jihadists’ biggest attack yet.

ISIS gunmen stormed a former US base in Tikrit, a city the group has held since June 11 and government forces have battled unsuccessfully to reclaim. The latest failed attempt marks what may be the army’s final push to reclaim the lost ground.

According to McClatchy DC, witnesses testified that by Friday the last of the government troops had surrendered to ISIS, and Camp Speicher had fallen. Officials in Baghdad declined to give comment, while the terrorist group boasted on Twitter about the losses the government had incurred. Statements from local residents corroborated the story.

The last pocket of government troops attempting to recapture Tikrit reportedly consisted of 700 troops, accompanied by more than a 100 Iranians, according to one local resident. He added that plenty of government equipment has been captured or destroyed in the process. The militants had previously captured millions of dollars in army equipment, which they paraded through every area they seized.

“They were being bombarded and mortared all night, and by Friday morning you could see burning helicopters everywhere and the fighting had stopped,” the resident told the news portal. Many executions were also allegedly carried out, while some prisoners were paraded through Tikrit – Saddam Hussein’s birthplace.

The campaign has been viewed as one of the most potent successes of the terrorist faction on the Iraqi battlefield.

Meanwhile, ISIS also appears to be on the ascendency in Syria. According to reports, at least 270 Syrian troops and civilians died Thursday at the hands of ISIS in Homs province, when a seized gas field was set ablaze, the Syrian Observatory for Human Rights reported. Like with Iraq, the watchdog is calling this the “biggest” success of the jihadist group in the country.

"A large majority of the men killed were executed at gunpoint after being taken prisoner following the takeover of the camp," the group’s director Rami Abdel said. "Eleven of the dead were civilian employees, while the rest were security guards and National Defence Forces members," he continued.

"Thirty martyrs were brought to Homs hospital from the Shaar gas field... Homs is still bleeding," a pro-government Twitter user was quoted by The Telegraph as saying. He called the events a “massacre,” although no official comment from the Syrian government was given.

The anti-Assad Observatory also reported on the deaths of 40 ISIS fighters in the offensive. About 30 people managed to escape the violence into the neighboring Hajjar field, according to the group.

There is talk of bloody footage filmed by ISIS, in which fighters pose with corpses. One fighter celebrating the killings speaks a mixture of German and Arabic.

The news was followed by condemnation from The Observatory.

"Summary execution is a war crime - whether of civilians or combatants. They are prisoners of war and must not be executed," Abdel Rahman said.

The latest violence comes on the heels of a UN report detailing the militant group’s activities. The document tells of 5,000 Iraqi deaths at its hands, many of whom were women and children.

The Islamic State has declared a “caliphate” in areas it captured in both Iraq and Syria, while the harshest and most unforgiving form of Islamic Sharia law has been imposed.

In Eastern Syria, in Raqqa, two women were stoned to death on Friday and Saturday for adultery, the report by the Observatory claims.

The death toll in the Syrian conflict, meanwhile, has climbed to 170,000 following three-plus years of fighting.