Thai and foreign rescuers continued Thursday to grapple with how to extract 12 boys and their soccer coach safely from deep inside a cave complex in northern Thailand in which they have been trapped for more than a week.

Chiang Rai Gov. Narongsak Osottanakorn, who is overseeing the rescue operation, told a press conference that the authorities are working intensively on an extraction plan.

However, he noted that while the water level in the cave is continuing to recede as a result of intense efforts to pump out floodwaters, the rate of drop has slowed to just 1 centimeter a day, compared to 4 or 5 cm at the start. Thailand is currently in the midst of its monsoon season.

Extracting the group through the cave has seemed like the only realistic option so far. But given that making their way out would take hours, even for experienced divers, rescuers are also considering accessing the part of the cave where the group is located from above through a shaft in the mountainside.

"We will try to find the exact place on the top that coordinates with the underground where the group is staying and see if it's possible to drill," the governor said, adding that a large number of survey teams will join the operation above the cave.

Rescuers are in particular searching for a hidden shaft said to extend to the so-called Pattaya Beach area of the cave, around 400 meters from where the boys are sheltering.

(A team of medics rehearsing their rescue operation for the 13 trapped in a cave.)

Narongsak said that the boys were given scuba diving lessons on Wednesday and it is up to the Thai navy SEALs to decide when they are ready to dive out.

"I let the SEALs and doctor evaluate the situation. If we have to bring them out even when the extraction is not 100 percent safe...we will evacuate them," he said.

Until now, the physical condition of boys has been given top priority, with stress placed on not rushing their evacuation. However, he indicated that if water levels rise again as a result of monsoon rains predicted for later this week, the extraction may be brought forward even if some degree of risk is entailed.

As for work to install a phone line for the boys to speak with their families, he said it is taking longer than expected, due to the narrow access in the cave.

"It takes around 11 hours to reach and return from the place where they are staying...the main obstacle of the extraction is the small and narrow channel," Narongsak said.

However, he said that morale is still high among the 12 boys, aged between 11 and 16, and their 25-year-old male coach, who went missing after entering the Tham Luang Nang Non cave in the province on June 23.

They were found deep inside the cave by two British rescue divers late Monday amid a frantic search effort by hundreds of rescuers and support personnel.

Meanwhile, Prime Minister Prayut Chan-o-cha said Thursday that King Maha Vajiralongkorn has issued a statement praising the efforts of Thai and foreign rescue teams. The king said the success of the operation is the result of collective cooperation and sacrifice of all sides.

"This circumstance has clearly shown the power of unity in action, power of love and goodwill towards fellow men regardless of race and religious belief," the king said in the statement.

As an expression of gratitude, the Foreign Ministry has sent letters to thank the governments of seven countries that provided rescue assistance.