Defence Secretary Philip Hammond has insisted the military campaign in Afghanistan will not be derailed as he paid tribute to the latest British soldiers to lose their lives.
Mr Hammond said two of the deaths were "all the more raw" because the killer had been dressed as an Afghan policeman and feigning injury.
The comments came as he updated MPs following a bloody spell for Nato forces in the troubled country.
Married father-of-two Sergeant Gareth Thursby, 29, and Private Thomas Wroe, 18, were shot dead in the south of Nahr-e Saraj district, Helmand province, on Saturday, in what appeared to be the latest in a string of "green-on-blue" incidents.
The soldiers, from 3rd Battalion, The Yorkshire Regiment (Duke of Wellington's), were killed at a checkpoint when a man dressed as a local Afghan policeman pretended to be injured so they would help him.
On Friday, Lance Corporal Duane Groom, from 1st Battalion Grenadier Guards, died after his vehicle hit a roadside bomb. On the same day, two US Marines were killed and six planes destroyed during an attack on Camp Bastion, the desert base in Helmand province where the bulk of the UK's 9,500-strong force in Afghanistan are deployed.
Taliban sources claimed Bastion was targeted because Prince Harry is serving there on his second tour of duty in the country as an Army captain. The Prince was about two kilometres away with other crew members of an Apache attack helicopter and was not harmed, and Mr Hammond has revealed Harry was moved to a guarded location when the attack began.
Answering an urgent question in the Commons, Mr Hammond said: "In recent days we have again been reminded of the difficult and challenging environment in which our armed forces operate. Our servicemen and women are doing vital work protecting the UK from the threat of international terrorism.
"Our strategy is clear - we are mentoring and training the Afghan army and police to deliver security to their own people... The Taliban hate this strategy and seek to wreck it through insider attacks. They aim to disrupt the collaboration with Afghan forces which is at the heart of our strategy. We cannot and we will not allow the process to be derailed."
Mr Hammond said the partnership with the Afghan security forces "involved risk but it is essential to success". He said the issue had been "at the top of my agenda" during recent meetings with president Hamid Karzai and he had been "reassured" that the Afghan government was doing everything it could to minimise the danger.