U.S. authorities are searching for a mother and her 13-year-old boy who has refused treatment for his cancer, after they did not show up for a court hearing.

A Minnesota judge issued an arrest warrant Tuesday for Colleen Hauser and stated that her son Daniel Hauser be immediately evaluated by doctors.

"It is imperative that Daniel receive the attention of an oncologist as soon as possible," the District Judge John Rodenberg of Brown County, Minn., wrote.

The court hearing was scheduled to determine the state of the boy's Hodgkin's lymphoma.

On Monday, Hauser and her son left their home in Sleepy Eye, Minn. sometime after visiting the doctor's office where they learned his cancer had in fact gotten worse.

During the hearing, Dr. James Joyce testified when he saw the boy and his mother in his office he discovered Daniel Hauser has "an enlarged lymph node" near his right clavicle and that an X-ray showed a mass in his chest was worsening.

The boy's father, Anthony Hauser, attended the hearing and testified that he was unaware of his wife and son's whereabouts. He said he last saw his son Monday morning and saw his wife briefly that evening when she told him she was leaving "for a time."

Hauser later told a reporter he wants them to come back.

"I'd like to tell them, you know, 'Come back and be safe and be a family again,'" he said. "That's what I'd like to tell them."

Rodenberg ordered custody of the boy be transferred to Brown County Family Services and issued a contempt order for the mother for her failure to appear.

Doctors said Daniel Hauser's first round of chemotherapy in February helped his condition.

But his parents felt that further radiation treatment would harm their son.

In a written statement issued last week, the parents' attorney stated they "believe that the injection of chemotherapy into Danny Hauser amounts to an assault upon his body, and torture when it occurs over a long period of time."

The Hausers, who are Roman Catholic, said they believe in the "do no harm" philosophy of the Nemenhah Band, a Missouri-based religious group, which uses some natural healing methods of the American Indians.

Colleen Hauser testified earlier that instead of further medical treatment she had been using herbal supplements, vitamins, and other natural alternatives to treat her son's cancer.

But the court found that the Hausers' action to take Daniel out of chemotherapy amounted to neglect and ordered the hearing last week.

Court documents show doctors estimated that further chemotherapy has an 80 per cent to 95 per cent chance of eradicating their son's cancer.

Since the disappearance of his wife and son, Anthony Hauser now agrees that his son should go back to the doctor for re-evaluation for the best treatment, said Calvin Johnson, a lawyer for the parents.

Philip Elbert, Daniel Hauser's court-appointed attorney, said he considers his client to have a "diminished capacity" because of the illness and his age and does not have enough information to make an informed decision about his treatment.

With files from The Associated Press and reports from CNN and ABC News