Ever inventive in its murderous ways, the Islamic State (ISIS) has found yet another method of executing its perceived foes.

Reports from Iraq say the terrorist group recently publicly executed 25 civilians by throwing them into a basin containing nitric acid in Iraq's embattled northern city of Mosul in Nineveh Province.

"ISIS terrorist members executed 25 persons in Mosul on charges of spying and collaborating with Iraqi security forces," IraqiNews.com quoted an anonymous source as saying.

"ISIS put the citizens in a large tub containing nitric acid inside one of its headquarters," the source added.

The source said ISIS militants tied all the 25 men with ropes and then lowered them in the tub "till the victims' organs dissolved."

According to the Chemical Company, nitric acid, or HNO3, belongs to the group of inorganic acids. It is both extremely corrosive and toxic. Thus, direct contact can result in severe burns.

Aside from adding more techniques in executing its captives, the ISIS is also adding new "crimes" punishable by death.

ISIS militants recently executed 11 other residents of Mosul on charges of using mobile phones, which is forbidden, according to the Alalam news site.

In Kirkuk city, the ISIS militants reportedly burned all five members of an Iraqi family alive a day after accusing them of trying to leave the city, another new "crime" punishable by death.

The ISIS has also reportedly forbidden civilians from fleeing the city of Raqqa in Syria, which the Kurdish forces are said to be preparing to recapture, Sputnik agency's sources said. Militants have allegedly begun removing their own families from the ISIS stronghold and de facto capital.

The report, citing sources on the ground, follows a U.S.-led coalition's air propaganda campaign urging citizens to leave the city.

Meanwhile, in late April, the ISIS executed 250 women in Mosul after they refused to get married to militants, Alalam said.

Last week, a mass grave of ISIS victims was found in the Suluk town in Syria, about 30 miles from Raqqa and which was controlled by ISIS until recently.

"We discovered thousands of bodies of innocent victims in the ravine," a Kurdish commander told RT.com. "They would bring those still alive to the precipice, blindfold them and shoot above their heads to scare them. People would then start running and fall over the edge. They'd bring the bodies and others, covered in blankets, and throw them off. The proof is these bones and the blood that's there. Nobody knows how deep the gorge is."