Norwegian channel NRK spoke to the father in Hatay, on the Syrian border with Turkey, on Tuesday night, as he tried to find his two daughters before they crossed into the war-torn country.
He said that he had managed to get through to the eldest daughter on her phone since she left Norway, and had received an email from them.
"Both in the email and the phone call, she told that she has traveled to Syria to participate in jihad and perhaps die there," he told he channel. "She is willing to die."
He said he believed that the two sisters, aged 16 and 19 years old, had been brainwashed.
"Both must have been brainwashed. I do not know by whom, but it must have happened in Norway. I call it brainwashing when you are persuaded by one or more people that you have to go to Syria to die there," he said.
The father contacted Norwegian police as soon as he received the email from the two sisters, who then put out a message on interpol.
The father told NRK that he believed the two girls were still in Hatay and had yet to cross the border, although the eldest had now stopped picking up her phone.
"She answered the phone when I called. I tried to convince her that what she had done was wrong," he said. "She interrupted me and said, "Dad, just forget it. It's too late. "
He said he believed that the daughters planned to help by doing humanitarian work, rather than fighting.
The family is not particularly religious, but the older girl recently became radicalised and started wearing a full-face veil, or niqab.
According to estimates from Norwegian intelligence, between 30 and 40 people have left Norway to take part in the Syrian conflict.