Live updates: Follow the latest news on Israel-Palestine
"This morning my daughter, Shani Nicole Louk, a German citizen, was kidnapped with a group of tourists in southern Israel by Palestinian Hamas," Ricarda Louk said in a video message shared by German media outlet Bild.
In the video, she holds up a photo of her daughter and later shares a photo of her passport.
“We were sent a video in which I could clearly see our daughter unconscious in the car with the Palestinians and them driving around the Gaza Strip. I ask you to send us any help or any news. Thank you very much.”
Israel's army has confirmed a "significant number" of people have been taken hostage by Hamas, which has been pummelled by Israeli air strikes since Saturday.
Shani Louk, who was attending a festival near the Gaza border, is widely suspected to have appeared in graphic video on social media, where Palestinian militants can be seen stepping on the body of a young woman in the back of a pick-up.
It is unclear if the woman in the video is alive.
Ms Louk's cousin told The Washington Post the missing woman's parents have watched the video and confirmed it was her.
Her boyfriend, a Brazilian, has also been confirmed missing, while other relatives told German media her credit card has been used in Gaza.
In a separate video, a woman identified as Noa Argamani, can be seen yelling "don't kill me" as she is driven toward Gaza on the back of a motorbike. An elderly woman was also seen being driven around Gaza in a golf cart.
This weekend has marked the worst Israeli-Palestinian violence in decades.
Almost 1,000 people have been killed so far, with no sign of an end to the conflict.
Al Qassam Brigades, the armed wing of Hamas, said it would provide details of the hostages later on Sunday and Israel has set up a command centre to trace all those who are missing.
A British-Israeli man has also been confirmed as missing after attending the music festival.
Jake Marlowe was working as part of a security team at the festival, his mother told Jewish News.
Mr Marlowe "called me at 4.30am to say all these rockets were flying over”, Lisa Harlowe told the outlet.
"Then, at about 5.30am, he texted to say, ‘signal very bad, everything OK, will keep you updated I promise you,' and that he loves me."
Israel's ambassador to Britain confirmed that British national was missing, but said it was not known whether he had been taken hostage.
Survivors of the attack on the festival have criticised the slow response from security forces.
"We were there for such a long time and no one was there. No army, no police. Nothing," Gilli Yoskovich told BBC News.
She said Hamas members were going tree-to-tree to shoot people.
"From two sides. I saw people were dying all around."
Several foreign workers are also believed to have been taken hostage.
Thailand confirmed two of its citizens were killed in the violence and at least 11 Thai workers are said to have been kidnapped.
Several Nepalis are also reported to be missing.