German federal prosecutors Tuesday announced a probe into Hamas on suspicion of kidnap and murder of German citizens, as the mother of a German-Israeli woman believed abducted appealed for help.

Prosecutors "have opened an investigation against unidentified members of the radical Palestinian Islamist organisation Hamas," a spokeswoman for the federal prosecutor's office told AFP.

The probe is over suspicions "of belonging to a foreign terrorist group, hostage-taking and murder", she said.

Dozens of foreigners have been killed, injured or taken hostage during the attack by Hamas.

A foreign ministry source in Berlin earlier said that several Germans who are also Israeli nationals were among those kidnapped by Hamas militants, without giving an exact number.

Chancellor Olaf Scholz said Berlin was working "intensively" with Israel to learn the fate of German citizens among the hostages and "what we can do to secure their freedom".

One German-Israeli woman, 22-year-old Shani Louk, is believed to have been abducted when militants attacked a rave party in the desert near the border with the Gaza Strip, according to her mother.

In a video on broadcaster ARD Tuesday, the mother Ricarda Louk said she had received new information that her daughter was "alive but has a severe head injury and is in critical condition.

"Every minute is critical, and we are asking the German government to act quickly."

Ricarda Louk first raised the alarm about her daughter at the weekend after she recognised her in videos circulating online, due to her striking tattoos and dyed hair.

The videos showed a half-naked woman lying seemingly unconscious face down in the back of a pick-up truck in Gaza filled with armed men.

Meanwhile a 22-year-old student from Berlin, who was on holiday in Israel, was killed at a kibbutz close to Gaza, her family and media reports said.

"We are trying to come to terms with losing Caro," her sister, Anja Pasquesi, wrote on Instagram late Monday.

"We are absolutely heartbroken."

Caroline's partner, Danny Darlington, is believed to be British, reports said.

German authorities have declined to make any comment on cases of Germans killed in the conflict.