The boys, aged between 11 and 16, and their 25-year old coach, went missing on Saturday while exploring Tham Luang cave complex that maze of passages and chambers is prone to flooding.
As a search in Thailand for 12 boys and their soccer coach missing inside a flooded cave entered a sixth day on Friday, police dropped survival packages through a shaft drilled in the mountainside not knowing if anyone was there.
The fate of the boys, aged between 11 and 16, and their 25-year old coach has gripped the country since they went missing on Saturday after they decided to explore the 10-km (6 mile) long Tham Luang cave complex in Chiang Rai province, despite warnings that the maze of passages and chambers is prone to flooding.
Bicycles and soccer shoes belonging to the boys were found near the entrance, and rescue workers think muddy handprints inside the cave could have been left by the group.
But the search has yielded no other trace.
International rescue teams, including one sent by the United States Pacific Command, are helping the Thai military and police in a search that has been hampered by heavy rain.
Police have been scouring mountain slopes above the cave in search of other ways in as divers tried to find their way through the flooded passages.
Twenty packages filled with water, food, medicine, flashlights and a note for the missing team were dropped down a fissure in the cave, police said.
Unsure of the boys' location, they hoped the boxes would reach them.
"If the children find this box we want them to float the box out of the cave," police Colonel Kraiboon Sotsong told reporters.
"The note says: 'If received, then reply and show on the map where you are. Everybody will quickly help'."
There has been an outpouring of support for the missing 13.
Prime Minister Prayuth Chan-ocha visited the site on Friday to urge on the rescuers and assure relatives keeping vigil that every effort was being made.
"Whatever can be done, do it, the government will back it," said Prayuth. "I've come to give everyone encouragement."
A group of school children has posted a video on Facebook featuring a song they wrote for the missing boys.
"Heavens please have mercy ... Please let them be safe, I pray to Mae Nang Non, please have mercy," the children sang, pleading to a deity, known as Mae Nang Non, who is believed to protect the Tham Luang cave.
Thailand is predominantly Buddhist and while it is hurtling toward modernity, animist beliefs and superstitions exert a strong influence over many people.
Several hashtags have appeared on Twitter in support of the 13 including "don't give up", "stay strong" and "13 Hope".
International soccer clubs, including the British teams Liverpool and Chelsea, have also expressed concern for the boys.
"Our hearts are in Tham Luang. We're praying for the boys and every rescue officer to return home safely. YNWA," wrote Liverpool, referring to its "You'll Never Walk Alone" anthem.