The loss of around two dozen elite commandos in a helicopter downed by the Taliban has dealt a major blow to US special forces, a key element in the strategy to wind down the Afghan war.

A navy special forces member interviewed by the Navy Times expressed "shock and disbelief," saying: "There's no precedent for this. It's the worst day in our history by a mile."

The Chinook transport helicopter was shot down by insurgents during a firefight southwest of the capital Kabul, according to Afghan authorities and the Taliban, killing 30 US troops, including around two dozen Navy SEALs.

Seven Afghan commandos and an interpreter were also killed in the attack, the deadliest for the NATO-led coalition since the invasion to oust the Islamist Taliban in the wake of the September 11, 2001 attacks.

US special operations forces have played an increasingly central role in the war, with thousands of members of elite units carrying out scores of commando operations -- usually at night -- to capture or kill senior Taliban fighters.

The low-profile units and surgical strikes are part of NATO's strategy of battling the Taliban while trying to minimize the impact of the war on the local population in order to win hearts and minds.

In the second half of 2010 some 7,000 operations, not all carried out by special forces, led to the killing of 2,000 insurgents and the capture of 4,000, according to NATO's International Security Assistance Force (ISAF).

US media reports have said the commandos killed in the chopper crash came from SEAL Team Six, a 300-strong unit considered the elite of the elite, which carried out the raid that killed Al-Qaeda chief Osama bin Laden in May.

Administration sources interviewed by AFP would neither confirm nor deny that Team Six or Navy SEALs were involved in the chopper crash, but they said none of those who took part in the bin Laden raid were among the casualties.

CNN said the troops in the chopper were part of a "quick reaction force," charged with swooping in to back ground troops under attack and in trouble.

Such units are often deployed on special forces operations and were present during the bin Laden raid, indicating the operation late Friday was aimed at an important target.

The Pentagon will struggle to replace such a large number of commandos, given that it takes five years to train them, according to Captain Kenneth Klothe, a special forces expert at the National Defense University (NDU).

The quickening pace of elite operations -- now reportedly carried out in Afghanistan, Iraq, Yemen and Somalia -- had raised concerns even before the helicopter crash.

"The problem is that it's very difficult to grow special forces overnight," Admiral William McRaven, the new commander of US special forces, said in June at a Senate confirmation hearing.

"One of the greatest challenges I think we will have for the future is there will be a greater demand on SOF (special operations forces)," he said.

There are currently around 140,000 foreign soldiers in Afghanistan, with about 100,000 of them from the United States.

All foreign combat troops are due to leave by the end of 2014, but intense violence in recent months, including a series of assassinations in the volatile south, have raised doubts about Afghan security forces.

"As the drawdown occurs in Afghanistan in terms of the conventional force, there will probably be some additional requirement for special operations forces to cover down," McRaven said.