The Islamic State (ISIS/ISIL) has killed at least three American volunteers so far this month who traveled to Syria to fight the jihadist group alongside U.S.-backed Kurdish troops.

Two of the volunteers had served in the U.S. military.

NBC News reports that the fatalities shine “a spotlight on a little-known underground pipeline of U.S. citizens who leave their lives and families behind to oppose ISIS in battle.”

ISIS jihadists killed the three Americans while they were fighting with the People’s Protection Units (YPG), the armed wing of the Kurdish Democratic Union Party (PYD) that controls large swathes of northern Syria.

YPG fighters are leading the U.S.-backed Syrian Democratic Forces (SDF), a coalition of Arab and Kurdish troops considered the American-led coalition’s principal ally against ISIS.

The SDF is primarily made up of YPG troops, which U.S. NATO ally Turkey has deemed a wing of the communist terrorist group known as the Kurdistan Worker’s Party (PKK), a position that has complicated the relationship between Washington and Ankara.

U.S.-backed SDF troops are currently fighting to liberate Raqqa, ISIS’s de-facto capital in Syria.

ISIS killed all three Americans in Syrian city — U.S. Army veteran Nicholas Alan Warden, 29, from New York on July 5; Robert Grodt, 28, from California on July 6; and U.S. Marine veteran David Taylor, 25, from Florida on July 16.

NBC News and the Associated Press confirmed the casualties.

“The presence of American volunteers with the YPG is complicated both militarily and diplomatically. The government of Turkey, an important NATO ally of the United States, regards the YPG as a terrorist group intent on carving out an independent Kurdish state,” points out NBC News. “Turkish forces have killed scores of Kurdish-affiliated fighters in airstrikes over the past year.”

Meanwhile, the U.S. considers the Kurdish-led SDF to be essential partners against ISIS in Syria.

The YPG has publicly praised the contributions of American volunteers to the fight against ISIS.

In a statement, the Kurdish militia said U.S. Marine veteran Taylor, who used the pseudonym Zafer Querecox and joined the group in May, was “martyred” and “loved among his comrades.”

“He had a big heart and was an expert at what he did,” added the YPG.

After serving two tours with the U.S. Army in Afghanistan, Warden joined the French Foreign Legion to take on ISIS affiliate Boko Haram in Africa, his father told NBC affiliate WGRZ of Buffalo.

Warden, who used the nom de guerre Rodi Deysie, reportedly joined the YPG in February.

In a YPG video, Grodt, an activist affiliated with the Occupy Wall Street movement, said he joined the militia “to help the Kurdish people in their struggle for autonomy” and to fight ISIS.

The Marxist YPG has attracted self-described communists and anarchists from the United States.

“They’re our anarchist comrades,” a U.S. resident identified only as 23-year-old Hristo from Brooklyn told New York City’s progressive newspaper the Village Voice before his failed attempt to enter Syria last year. “I feel it’s my obligation to go aid them.”

He revealed that many American volunteers have to illegally sneak into Syria to join the Kurdish group.

According to the Voice, Guy McGowan Steel Steward, 22, of Manhattan became “the first reported American [last year] to join the leftist group.”

The Voice reported that at the time “more than 100 American citizens” had traveled to Iraq and Syria to fight alongside the Kurds.

Most of the Americans “have been military vets, Christian crusaders, or adventurers looking for thrills and a chance to kill ISIS fighters,” noted the newspaper.