A desperate rescue operation is underway to save a toddler who fell more than 100 metres down a narrow well in Spain.

Hundreds of emergency workers are at the scene, but the shaft’s narrow opening – just 25 centimetres in diameter – has prevented rescuers from descending down the shaft.

The boy, who slipped and plunged down the 107-metre well in the area known as Dolmen Cerro de la Corona, was playing with a young family member while his parents were at a nearby farm.

Yulen reportedly let out a scream upon falling but has not been heard since.

The fire department, Civil Guard, Local and National Police and emergency health services were immediately alerted to the incident after an alarm was raised.

Rescuers lowered a micro-robot with cameras about 80 metres down the well, but have so far been unable to locate the child, only retrieving a bag of sweets that the boy had reportedly been carrying.

Ads the rescue efforts reached their 18th hour, emergency services were considering “all alternatives” to free the boy and workers planned to dig a hole parallel to the well in an effort to gain access to the shaft.

“We haven’t found or heard the child, that is why we are taking the time we need to take to find him,” Civil Guard spokesman Bernardo Moltó told CNN.

“Our number one priority is to find and rescue the child,” Mr Moltó said.

But emergency personnel hold grave fears the boy may have drowned into a mixture of sand and water at the bottom of the well.

Mr Moltó also warned that upon the boy’s descent down the well, the walls of the well may have caved in on the child.

The well, originally dug as part of a water exploration project, was meant to be sealed over, but its cover was removed and lying nearby, causing the boy to trip and fall.

Authorities will complete a full investigation into the incident.

According to local newspapers, Yulan’s older brother Oliver, aged three, died suddenly of a heart attack in 2016.

-with agencies