A German woman fighting with Kurdish militiamen was killed battling ISIS in Syria, Kurdish officials said Monday — the third foreign national known to be killed fighting with Kurdish forces against the extremists in the war ravaged country.

Ivana Hoffmann, 19, died Saturday while fighting alongside the Kurdish Peoples Protection Units, known as the YPG, near the Syrian village of Tel Tamr, YPG spokesman Nawaf Khalil said.

Elsewhere in Syria, airstrikes by the U.S.-led coalition hit an oil refinery held by the Islamic State group outside the town of Tel Abyad, activists said. Video from the Turkish Dogan News Agency showed the strikes Sunday night, which resulted in an enormous fireball that engulfed the refinery.

The Britain-based Syrian Observatory for Human Rights, which relies on a network of activists inside Syria, estimated that the strikes killed about 30 people — including ISIS militants and refinery workers. The Syrian activist group known as Raqqa is Being Slaughtered Silently also reported the strikes, but offered no casualty figure.

The ISIS group, which controls a self-declared caliphate on captured territory covering about a third of Syria and Iraq, partially funds its conquests through the sale of black market oil. The U.S.-led coalition did not immediately acknowledge launching the strikes.

Hoffmann, a member of the Marxist-Leninist Communist Party (MLKP) in Turkey, joined YPG fighters about six months ago, according to a statement released by the MLKP.

The party statement referred to Hoffmann by her nom de guerre, Avashin Tekoshin, and said she died in pre-dawn clashes with ISIS militants on March 7.

"Our comrade Avashin had been at the front using her weapons to resist the bloody onslaught of the IS gang against the Assyrian villages in Tel Tamr for days," the statement said. "During these clashes, dozens of gang members were killed. Our comrade Avashin fought to the last bullet together with the fighters of the YPG."

The party statement did not mention how many other MLKP loyalists have travelled to Syria to fight ISIS. German authorities say some 650 people have travelled from Germany to Syria and Iraq, to join Islamist groups, but they haven't said how many are estimated to have joined Kurdish or Christian groups opposing ISIS. The head of Germany's military intelligence agency, Christof Gramm, told daily Die Welt in an interview published Monday that about 20 former German soldiers had travelled to the conflict zone.

Hoffmann, born in Germany to South African parents, is the third Westerner — and the first female foreign fighter — known to be killed fighting with Kurdish forces against the Islamic State group.

A video posted early Monday morning on a Facebook page memorializing Hoffmann shows a woman with her face covered by a scarf holding a weapon and speaking German. She refers to Daesh, the Arabic acronym for ISIS, and to Rojava, a Kurdish word that denotes the now largely autonomous areas in north and north-eastern Syria run by Kurds.

"Behind us is the territory of Daesh. We've been here for a week. For one week we've been holding our base to defend the Rojava revolution. I decided to come to Rojava because they are fighting for humanity here, for rights and for internationalism that the MLKP represents. We are here as the MLKP to fight for freedom. Rojava is the beginning. Rojava is hope," she said.

A German Foreign Ministry spokeswoman, Sawsan Chebli, said she was unaware of reports about Hoffmann's death.