Hundreds of people have been mobilised to find the young football team and their coach who went into the cave on Saturday and were trapped when heavy rains are believed to have flooded the cave's main entrance.
Anxious relatives camped out at the cave near the border with Myanmar and Laos praying for the children's safe return.
"My child, I'm here to get you now," one crying parent said, according to footage from local media.
The children, aged between 11 and 16, are believed to have retreated further into the cave as monsoon rains fell and flooded the cave, believed to be several kilometres (miles) long.
Park officials, police and soldiers were dispatched on foot, while an aerial team was also being prepped to scour the scene, according to an AFP photographer at the scene.
Navy SEAL divers equipped with oxygen tanks and food rations entered the cave early Tuesday in northern Chiang Rai province, where light rains continued to fall.
An underwater robot is set to be dispatched later Tuesday to help survey the Tham Luang cave, a popular site among local tourists.
Rescuers found bicycles and a motorbike at the entrance of the cave Monday, and divers said they spotted footprints in one of the cave's chambers.
Heavy rains are common in the area during Thailand's monsoon season from May to October, with flooding and landslides routinely reported.
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