Police have launched a major helicopter search after two young children, thought to be aged about four and six, reportedly spotted walking alone in Hertford Heath.
Hertfordshire Constabulary posted the emergency alert after the children were seen on London Road around 9.45pm on Sunday.
It's thought that a boy, around four or five, was walking towards Hoddesdon with a young girl, aged six or seven.
Police say a caller reported that they were both in dark clothes.
In a Facebook post, the constabulary said: "We are currently searching Hertford Heath after a resident reported seeing two very young children walking towards Hoddesdon, apparently without an adult.
"The caller said they had seen a boy, aged four or five, and a girl, around six or seven years old, both wearing dark clothing, walking along London Road.
"They alerted police at around 9.45pm and specialist teams are now in the area, including the police helicopter.
"If you have any information, please call 999, quoting ISR 648."
