Four more boys have reached Chamber 3 in Tham Luang -- past the narrow, treacherous passage near the T-junction that poses the greatest threat to the rescue operation, a source in the operation centre said on Monday.

The source said the boys arrived at the chamber at 4pm after being rescued by foreign and Navy Seal divers from the ledge where they had sheltered from floodwaters for more than a week.

They are the second batch to undertake the perilous journey out of Tham Luang cave in Mae Sai district in Chiang Rai. The first four were successfully evacuated and taken to Chiangrai Prachanukroh Hospital on Sunday.

Divers took about four hours to escort the four from the ledge to the chamber, the source added.

Chamber 3 is the operational base for rescuers inside the cave. It is about two kilometres from the entrance.

The evacuation procedure calls for the boys to receive initial medical treatment there before rescuers take them through the remaining stretch of the cave to a field hospital outside, and then transport them by helicopter or ambulance to join their colleagues at the hospital.

Rescue mission chief Narongsak Osotthanakorn on Monday said the operation on the second day started at 11am.

Four more young footballers and their coach at the Mu Pa (WIld Boar) Academy Mae Sai, Ekkapol Chantawong, are still on the ledge.

The thirteen were trapped by floodwater after entering the cave on June 23, and forced deeper and deeper inside the labyrinthine tunnels.