Defence analysts are considering the causes and implications of an aircraft crash that killed 14 British service personnel in Kandahar in southern Afghanistan.
A technical fault is believed to have caused the crash
Defence analyst Paul Beaver said it was unusual for a Nimrod to be involved in an accident.
"For an aircraft this large to have a technical fault, which is what it seems to be, is unusual.
He said it was usually a "very safe aeroplane", which would make the incident come as even more of a shock.
Correspondents said the incident was likely to raise more questions about the Afghanistan mission.
BBC defence correspondent Paul Wood said: "A lot of people hoped that it would be a peace-keeping, nation-building mission instead it is an active, counter-insurgency campaign."
He said the Conservative front bench had been supportive of the presence of troops in Afghanistan and Iraq, but he said, others were "going back to those basic questions - what is the mission in Afghanistan?
"Is it an achievable mission especially when you are trying on the one hand to fight guerrillas and on the other opium - can those two missions be achieved side by side?"
Our correspondent added the other key question being raised was "what was the exit strategy?"
He said there were likely to be more losses.
"The Taleban has shown itself, in the words of one senior officer - willing to commit its own forces extremely recklessly - and that sadly will mean British casualties."
However, he added the crash did appear to have been caused by a mechanical failure and "people can discount" the Taleban's claim to have shot it down as it was a claim they routinely made for "propaganda purposes".
He said it remained to be seen following the crash investigation if there were "any special pressures or stresses" associated with the Afghanistan operation linked to this crash.
But he said such questions over pressures "would certainly" have been raised had the aircraft involved been a Chinook helicopter.
The defence editor of the Evening Standard, Robert Fox, said the situation was now "very serious indeed".
"Whatever the circumstances of what has happened to this plane, it is very, very difficult for Britain to sustain losses of this level.
"It's difficult because there have been so many questions raised about this operation from the beginning including very senior, recently retired officers writing personally to the prime minister and ministers saying please think again."
He said the "general consensus" was that resources, in terms of troops and aircraft, were being overstretched.
"We only have about 700 combat troops at work in Afghanistan, in an area as large as Wales," he said.
"And where there is an enemy that is fighting that has been completely underestimated."
Defence analyst Major Charles Heyman told News 24 it was a "black day" for soldiers on the ground in Afghanistan.
"Believe me it really really does affect morale," he said.
He added the deaths would also affect public opinion.
"People are going to say 'what are our troops doing there?'
"Overall, it begins to look like an unlucky operation. There have been a lot of deaths."