On damp scraps of paper torn from an exercise book, brief but heartbreaking messages from the boys trapped in a Thai cave offer a slender ray of hope for their desperate parents on the surface.

The chirpy messages of love for their families and longing for their favourite foods are in stark contrast to the darkening mood and sky outside.

The past few days have been relatively dry but with today's forecast of monsoon rain that could flush through the Tham Luang Nang Non cave 'like a tsunami' comes the terrifying realisation that time is running out.

Speaking outside the cave, Governor of Chiang Rai province Narongsak Osatanakorn, who is in overall charge of the rescue, said: 'We're trying to pump water out, but more and more is coming in from above and below. Our biggest concern is now the weather.

'A heavy downpour comes down into the caves like a tsunami.'

He added: The children are learning how to dive. We'd like minimum risk, but we can't wait until it rains heavily and worsens the situation.