The head of the Army has defended Britain's military strategy in Afghanistan amid claims that troops were "on the brink of exhaustion".
General Sir Mike Jackson said the Army played a vital role and forces were "getting stuck in" to the Taleban.
It comes after a senior officer told the Sunday Telegraph troops were very tired after fighting 25 major battles since May in temperatures of up to 50C.
Nine British soldiers have been killed in Afghanistan in the past two months.
They died during operations in Helmand province, in the southern part of the country.
Captain Alex Eida, 29, 2nd Lieutenant Ralph Johnson, 24, and Lance Corporal Ross Nicholls, 27, who died last Tuesday, were the latest victims.
The source told the Sunday Telegraph: "The men are knackered - they are on the brink of exhaustion. They are under considerable duress and have suffered great hardship.
"This is a situation which is ultimately unsustainable. The shock of battle, the lack of sleep and back-to-back operations are beginning to impact on the troops.
"They are now close to what is realistically achievable - even for the Paras."
The newspaper reported that 700 troops were bearing the brunt of the heavy fighting and said commanders wanted the overall 3,600 force to be supplemented by another 1,000-strong infantry battle group.
This would be over and above the extra 900 soldiers recently committed.
Terrorism safe haven
Meanwhile, military expert Maj Charles Heyman warned that British forces faced a potential situation which could see isolated outposts struggling to hold off Taleban fighters.
"There is a little bit of Rorke's Drift about this and if we are not very, very careful we could have a disaster on our hands," he said, referring to the famous 1879 encounter when a British force of just over 100 defended a garrison in South Africa against more than 4,000 Zulu warriors.
Sir Mike, who is due to retire at the end of this month, told BBC News: "We, the international community, must be doing what we're doing in Afghanistan.
"We cannot afford to let that country go back to where it was, as a safe haven for international terrorism. That is simply unacceptable. We have to do this.
"We are doing it as part of an international force."
And, when asked about foreign armies in the past becoming bogged down in Afghanistan, Sir Mike said he was "well aware" of the military history of the country.
He added: "The other side of that coin is that in which case, don't do this at all, let the Taleban take over Afghanistan again, let al-Qaeda have another safe haven there. This is not the stuff of strategy."
Meanwhile, a Ministry of Defence spokeswoman said: "The force package and troop numbers are those requested by commanders on the ground.
"We keep all our operational requirements across the world under constant review."