MELBOURNE, Fla. -

An alert has been issued for two missing children whose parents are believed to have a pact to kill them, according to authorities.

The girls are two and four-years-old and were last seen in Melbourne, Florida.

Melbourne police said the family resides in Vietnam, but came to the U.S. to treat their four-year-old for autism. Witnesses told police the suicide pact was a result of the father's frustration with the expense of treatment, authorities said. According to police, the parents said they would kill themselves and the kids if they ran out of money.

Kiah Firth was last seen wearing an all pink sundress with flower print. Her sister Kristin Firth was last seen wearing a dark blue sundress with a belt and a bowtie in the back.

Authorities said the children were last seen in the 100 block of West University Boulevard in Melbourne.

The children may be in the company of their parents, James and Mai Firth. The father was last seen wearing a black V-neck sleeveless shirt, black pants and black shoes, authorities said. The mother was last seen wearing a multicolored flowered dress and a black coat, authorities said.

The group is believed to be traveling in a 2002 silver Lexus SUV with FL tag number 612PGW.

Anyone with information is asked to call the Florida Department of Law Enforcement at (888) FL MISSING or the Melbourne Police Department at (321) 409-2205 or 911.