LONDON -- Two American men have been killed fighting against Islamic State of Iraq and Syria (ISIS) militants as the terror group clings to its last urban stronghold, the terror group's de-facto capital city of Raqqa in Syria.

One of the most potent forces battling ISIS on the ground in Syria -- with backing from the U.S.-led military coalition -- is the Kurdish militia known as the YPG. Volunteers with varying motives from around the world have joined battle-hardened YPG fighters during the last three years, often paying with their lives.

In a statement posted online this week, the YPG identified two U.S. nationals, both men, who it said were killed in the battle to retake Raqqa from ISIS.

Robert Grodt, 28, a former "Occupy Wall Street" activist, appeared in a YPG YouTube video only two weeks earlier, explaining why he left his family and friends in New York to join the fight against ISIS.

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He said he was motivated to fight for the Kurdish "revolution" by a "buddy" who had joined in 2016. Like all foreign fighters who join the YPG, Grodt, adopted a Kurdish moniker, calling himself Demhat Goldman.

"I talked with my partner and my family, and I'm like, I'm gonna go out to Syria. This is something I care about," the California native said in the video, holding a rifle in one hand. He said he couldn't sit idle after, "seeing a real chance to take a hand in something that's really important for the region."

The Kurdish force said Grodt was killed in an ambush by ISIS militants on July 6, in a Raqqa suburb.

The YPG said the other slain U.S. national was Nicholas Warden, 29, who had adopted the Kurdish name Rodî Deysie. According to the statement posted by the YPG, he was killed on July 5, also fighting around Raqqa.

Warden's father Mark, who spoke emotionally on Tuesday to The Buffalo News from his home in Depew, New York, said his son was a U.S. Army veteran.

"He was very strong-willed and very strong-minded and very much against ISIS and these terrorist groups," Mark Warden told the newspaper of his son. "He wanted to do whatever he could to get rid of them. He said not enough people are helping so he had to help."

#YPG's American volunteer Nicholas Alan Warden (Rodi Deysie) was martyred in action July 5 while valiantly fighting Daesh (#ISIS) in #Raqqa. pic.twitter.com/hYczabNmsm — Rojava Defense Units (@DefenseUnits) July 11, 2017

In another YouTube video, similar to the one posted by the YPG with Grodt, Warden said he joined the Kurdish fighters "to fight ISIS because of the terrorist attacks they were doing in Orlando, in San Bernardino, in Nice (France), in Paris."

Nicholas Warden, of Depew, New York, is seen in a screengrab from a video released by the Kurdish YPG militia, which he joined to battle ISIS in Syria. YouTube/YPG

The two men, described as "martyrs" by the YPG, are not the first Americans to perish alongside Kurdish forces in the ISIS fight.

Almost one year ago exactly, Susan Shirley of Colorado said she received a call from the U.S. consulate in Turkey, informing her that her 24-year-old son Levi had been killed on the battlefield of Syria fighting with the YPG.

Shirley said her son had always wanted to serve in the U.S. military, but was unable due to poor eyesight. He died on his second stint fighting with the Kurds, after returning home but failing to adjust to life working fast-food jobs.

"He saw ISIS as a terrible evil, and that just was not okay with him," Shirley said. "That's the way his mind works. If you are defenseless, he will help you."

In 2015, Keith Broomfield, a 36-year-old Massachusetts man with no military training became the first U.S. citizen killed fighting ISIS alongside the Kurds.

Kurdish commanders said Broomfield died in battle near the northern city of Kobani.

His parents eulogized him as having given his life "in defense of those being persecuted for their Christian faith" and "for the protection of the innocence of Kurdish women and children."

A British man, Luke Rutter, was also killed in the July 5 ambush alongside Grodt, the Kurds said in the statement released on Tuesday, along with several native Kurdish fighters.