The president of Fifa has invited the 12 boys and their football coach trapped in a flooded cave in Thailand to attend the World Cup Final in Moscow - if they are rescued in time.

Thai authorities are still weighing up different options to free the young footballers in Chiang Rai, who have now been stuck in a small chamber, about 1km underground, for almost 13 days.

The World Cup Final is due to take place on Sunday 15 July at the Luzhniki Stadium. Gianni Infantino, the president of Fifa, said the organisation had been “anxiously following the news of the rescue operation”.

In a letter to the Thai Football Association, Mr Infantino wrote: “On behalf of the international football community, I would like to join you in expressing my deepest sympathies and support to the families of the players and coach, as well as my solidarity with the people of Thailand at this time of great concern.”

The Fifa president said he “hope[s] that in some way our words of support may help bring them a little peace and courage in these difficult moments of uncertainty and concern”.

He added: “If, as we all hope, they are reunited with their families in the coming days and their health allows them to travel, Fifa would be delighted to invite them to attend the 2018 World Cup final in Moscow as our guests.”

The Thai authorities overseeing the rescue have previously refused to rule out the option of having the boys simply wait in the cave until the floodwaters clear naturally, a process that could take months until the end of the monsoon season.

But on Friday, concerns were growing that worsening weather and lowered oxygen levels in the underground complex may force them to extract the boys in a difficult and dangerous diving operation.

The challenges of the situation were highlighted overnight by the death of a volunteer diver helping the rescue effort, former Thai navy SEAL Samarn Poonan. He fell unconscious underwater on his way back from dropping off oxygen supplies halfway along the route to the boys.

"We can no longer wait for all conditions (to be ready) because circumstances are pressuring us," Thai SEAL commander Arpakorn Yookongkaew told a news conference. "We originally thought the boys can stay safe inside the cave for quite some time but circumstances have changed. We have limited amount of time."

Other members of the international footballing community have also sent messages of support to the boys. The Japanese national team, shortly after they were knocked out of the World Cup by Belgium, urged the boys in Japanese: “Don’t give up! Hang on!” and in English “Hang on! Football Family are with you.”

After a rescuer filmed one of the boys wearing a red England football shirt. England defender John Stones said “we’re hoping and wishing that they get out safe and sound.”

“Everyone, their friends and families, all of the boys (here), all the England squad, all our thoughts are with them,” Stones said.