After hours of ferocious fighting in southern Afghanistan, the two young US Marines desperately needed emergency medical care -- and it was the heat, not the Taliban, that had finally defeated them.

Charles Auge and Edwin Saez had landed at a canal junction at dawn last Thursday as part of a major US offensive against Islamist insurgents in the key province of Helmand.

They were engaged in an intense battle through the heat of day against dozens of gunmen who were determined not to lose control of the Mian Poshtey intersection in the south of Garmsir district.

When their two-and-a-half-litre (five-US-pint) water backpacks ran out, Auge and Saez looked to restock from the bottles that Echo company from the 2/8 infantry battalion had brought with them on the helicopter assault.

But as the company came under constant fire, the supplies were limited and the water scorchingly hot when it did arrive.

"We were on the flank beside a thick grass berm, and in the middle of the day the sun was so strong and there was no shade," Auge, 24, said. "I began to feel dizzy and everything turned white."

Saez, 21, also became a "heat casualty" soon after, having shot at -- and apparently killed -- two gunmen who were firing at the Marines from behind a wall.