At dawn a long line of US armoured vehicles prepared to leave Forward Operating Base Spin Boldak for a high-risk mission into uncharted territory.

But they met a different kind of enemy from the one they were expecting.

The troops were on reconnaissance in the remote south, along the border with Pakistan, which is largely unknown terrain for NATO forces because only the Afghan border police operate here.

At the front of the convoy, Mine Resistant Ambush Protected (MRAP) vehicles equipped with reinforced V-shaped bodywork to deflect explosions were tasked with opening the route for the more lightly armoured "Stryker" vans.

Progress was expected to be slow but impossible without the MRAPs: improvised explosive devices (IEDs), the weapon of choice for the Taliban, are the primary cause of international troop deaths.

"It's going to take us 12 hours," said Staff Sergeant John Jenkins from infantry company Charlie 2-1.

But at 9:00 am, destination confidential for security reasons, the latter half of the convoy still hadn't moved. One of the MRAPs was stuck in sand near the base.

After four hours waiting, the heavily armed Strykers finally moved.

Inside the cramped and dusty Strykers soldiers put tourniquets in their pockets. In the event of an explosion they can stem the flow of blood from a life-threatening arterial bleed.

During the journey it was difficult to see what was going on outside, then the convoy stopped in the middle of a desert, where just the occasional shabby house dotted the parched land.

A logistics vehicle had technical problems, exasperating troops in one of the eight-wheeled Strykers.

As repairs were carried out, the young soldiers ripped open khaki-coloured plastic bags containing their food rations -- beef sandwiches with chocolate and peanut butter bars.

The convoy was brought to a halt again at about 5:00 pm. This time it was another MRAP, a 25-tonne "Buffalo" equipped with a mechanical arm to pick up IEDs, that broke down.

"These vehicles were not really made for the terrain here. They are more designed for Iraq," said Jenkins.

Iraq is where the 4,000 troops from Stryker brigade were supposed to go before learning they would instead be deployed straight to Afghanistan.

"I studied Arabic for a year," said Specialist Randy Kuykendoll. "Now I can sell a TV in Arabic but I've only had two months of Pashto."

As night fell and the temperature dropped to freezing, the convoy stopped and formed a defensive circle in a vast stretch of desert.

The US troops unfolded camp beds, took off their weapons and body armour and curled up in sleeping bags.

At daybreak, they headed back to base.

No trace of mines or insurgents were found. Instead, logistical problems ended the operation, infuriating the soldiers, who were eager to do battle with the enemy.

"The terrain is a lot more challenging than we expected," said Lieutenant Colonel William Clark, head of the 700 US troops based at FOB Spin Boldak.

"All our Strykers were fine but the engineers' equipment is more for a highway type of terrain."

Asked about the convoy's technical problems, Kuykendoll said: "I guess they're handling it as if it were Iraq."

Sergeant Jon Belajac added: "I can't imagine what it's going to be like when it starts raining."