CHIANG RAI, Thailand, June 29 (Xinhua) -- International humanitarian support teams from the United States, Myanmar, Laos, the Philippines and China have offered to help Thailand search and rescue 13 adolescent males who are currently trapped inside a mountainous cave in Thailand's northern province of Chiang Rai.

Around Tham Luang Khun Nam Nang Non cave in Mae Sai district of Chiang Rai, about 860 km north of Bangkok, the rescue mission has been going on around the clock after the 12 teenagers, aged between 11 and 16 years, and their 25-year-old soccer coach entered the cave on Saturday and were believed to have been trapped inside since.

Almost constant, heavy rains have thwarted the rescue mission which has become more urgent, complex and raced against time under all kinds of weather in a sixth day.

On Thursday, the Thai authorities said that they believed the rescue members could have reached a large, dry cavern, and officials were considering drilling through the mountaintop to reach them.

Meanwhile, some groups of rescuers are working hard on trying to find some shafts on the mountain, where they might be able to send people into the cave vertically. However, the mountain area sometimes can become very foggy, hindering a helicopter from flying above it.

Water pumps have been installed to drain water out, with five having been installed inside the flooded cave.

Continuous rains is one of the biggest obstacles for the rescue work, as it keeps the water level inside the cave relatively high. Thai Navy SEAL frogmen who have dived in the flooded cave are needing support from professionals of cave diving as three cave diving experts from the United Kingdom came on Wednesday.

The Navy Seal divers have made a temporary base camp about three kilometers from the entrance and staying inside for four days with a support team bringing gas cylinders for them on hourly basis.

The U.S. army also arrived on Thursday, with the type of equipment and plans being unknown as yet.

Thai Prime Minister Prayut Chan-o-cha arrived to the scene to direct the rescue mission by himself, thus keeping members of the media from getting very close to the cave.

Near the entrance, relatives of the missing teenagers have gathered, praying for their safety. One of the parents who asked not to be identified said that she still believed the boys are alive and all she could do was to pray and have faith.