CHIANG RAI, Thailand (Reuters) - A heavy rainshower hit northern Thailand late on Saturday, worsening conditions at the cave where rescuers were waging a “war with water and time” to save 12 trapped boys and their soccer coach, with a rescue possible within days.

Relatives, of the 12 schoolboys and their soccer coach trapped inside a flooded cave in the Tham Luang cave complex cook, for rescue workers and volunteers near the cave complex in the northern province of Chiang Rai, Thailand, July 7, 2018. REUTERS/Soe Zeya Tun

The rain could potentially set back progress made over the last week to drain the Tham Luang cave complex in the northern province of Chiang Rai.

With oxygen levels dropping and rainstorms forecast, time is running out on a plan to teach the boys -- some as young as 11 and not strong swimmers -- to make a dive through narrow, waterlogged passageways that would challenge experienced cavers.

The best window for a rescue could come in the next three or four days, the leader of the rescue effort said on Saturday.

Narongsak Osottanakorn’s cautious message of hope came a day after the death of a Thai rescue diver, a grim turn in what began two weeks ago as an adventure to celebrate the birthday of one of the boys.

Up on the hill, where rescuers are seeking alternative routes down into the cave, another accident occurred on Saturday night: a vehicle skidded off a dirt track, seriously injuring the driver and a passenger and harming four other volunteers, authorities said.

Earlier, at the sprawling cave mouth below, lines of frogmen and soldiers with flashlights could be seen emerging from the darkness, as generators chugged and pumped water out from blue nylon pipes.

Dozens of Royal Thai Army soldiers rested on rocks outside the cave, with two of them saying they had been told by superiors that the rescue operation would likely begin Sunday or the day after.

Authorities tightened a security cordon, draping plastic sheets around the approach to the cave, further giving the sense that a rescue might be imminent.

This could not be independently corroborated by Reuters, but Narongsak, a former provincial governor, told reporters earlier on Saturday that the next three to four days was “the best and most ideal time for the rescue operation.”

“The current situation, with the air and water levels and the boys’ health, is the best yet,” he added. “We’re still at war with water and time.”

The discovery of the boys inside the cave by British divers on Monday “was just a small victory”, he said.

For the first time since they were found, the boys communicated by letter with their relatives -- many camped outside the cave’s entrance. The boys wrote with both homesickness and humor.

Billionaire entrepreneur Elon Musk tweeted on Saturday that an escape pod and inflatable tube he was developing might be used to help the rescue. He gave no details. The Thai defense ministry said a team from a Musk firm with drilling and exploration knowhow should reach the cave on Sunday.

Several monks in orange robes sat at a small shrine with two golden deer statues near the cave mouth, chanting, as a middle-aged woman knelt and prayed before them.

An assistant told Reuters the ceremony was to “open up” the cave mouth to allow for an easy and safe return for the boys.

“DOUBLE POSITIVE”

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Ivan Katadzic, a Danish diving instructor who has been ferrying oxygen tanks into the cave, said after a dive on Friday he was “double positive” about the mission because the water level had dropped considerably.

Katadzic has not dived the final kilometer to where the boys are stranded on a muddy bank, the most dangerous part of the dive, during which rescuers have to hold their oxygen tanks in front of them to squeeze through submerged holes.

Alternative rescue plans include stocking the cave with supplies and an oxygen line to keep the boys alive for months until Thailand’s monsoon season ends, or drilling a shaft down from the forest above.

Narongsak said the drills would have to pierce 600 meters (1,970 feet) of fragile limestone rock to reach the boys and rescuers were discussing drilling angles.

Besides looking for possible holes from above, the team on the hill above is trying to block holes and divert streams that channel water into the cave before the weather turns.

“Everything is a race against time,” said Kamolchai Kotcha, an official of the forest park where the cave complex is located. His team would camp out on the hill to try and finish its work before the rain came, he said.

(Graphic: Hope for the 13 - tmsnrt.rs/2KR2zRj)