Packages have been dropped through a shaft in a mountainside in the hope of reaching 12 boys and their football coach trapped in a cave in Thailand.
The boys, aged between 11 and 16, and their 25-year-old assistant coach went missing on Saturday when they explored the six-mile Tham Luang cave complex in Chiang Rai province.
Bicycles and football boots belonging to the boys were found near the entrance of the cave.
International rescue teams, including the United States Pacific Command (PACOM), are assisting the Thai army, navy and police in the search operation which has been hampered by heavy rain.
Police have been searching ground above to find other ways into the cave as divers try to find their way through the flooded passages.
Twenty packages filled with water, food, medicine, torches and a note addressed to the missing team were dropped down a fissure in the cave on Friday.
But police are not sure exactly where the boys are.
Colonel Kraiboon Sotsong said: “If the children find this box we want them to float the box out of the cave.”
“The note says: ‘If received, then reply and show on the map where you are. Everybody will quickly help.”
Prime Minister Prayuth Chan-o-cha visited the site on Friday to provide encouragement to the rescuers and assure relatives keeping vigil that every effort would be made to find the boys.
He said: “Whatever can be done, do it, the government will back it.
“I’ve come to give everyone encouragement.”
At least three rescue workers were taken to hospital on Friday after being electrocuted. It is not known how serious their injuries are.