Gordon Brown said the latest deaths were a "tragic loss"
Gordon Brown is to make what advisers say will be a major speech on Afghanistan on Friday, amid growing calls for a rethink of UK strategy.
The deaths of five British soldiers at the hands of an Afghan policeman have raised questions about security and the UK's role in training Afghan personnel.
Ex-Labour minister Kim Howells has urged UK troops to be withdrawn.
But Armed Forces Minister Bill Rammell said there was "no credible alternative" to the present strategy.
Vetting procedures
Gordon Brown told MPs on Wednesday that the deaths of the soldiers - taking the total number of British personnel killed since 2001 to 229 - were a "tragic loss" and the circumstances would be urgently investigated.
The Taliban had claimed responsibility for the attacks, suggesting that militants may have infiltrated the Afghan national police force, he added.
No 10 has said that British commanders will be reassessing vetting procedures for applicants for the Afghan police as a result.
We cannot succeed in Afghanistan
Labour MP Paul Flynn
The UK is playing a lead role in training and mentoring Afghan police and army units on behalf of the Nato force in the country.
Ministers say the task is vital to enable more responsibility for security to be handed to the Afghan authorities, at which point the UK can begin to reduce its troop numbers.
Mr Rammell said such an aim was "achievable" and that there was no "credible alternative strategy".
He rejected suggestions from Mr Howells that the UK was better off focusing its efforts on domestic security and counter-terrorism and supporting the intelligence services in their battle against extremists.
"If we left precipitously tomorrow, I have no doubt that the Taliban would come back, there would be massive instability and we would be less safe in this country," he told the BBC.
He added: "I am not saying we should be there for ever. That is why we have to build up the Afghan capability so they can take up the responsibility themselves."
Ministers were doing everything they possibly could to ensure troops' safety, he added, promising that "lessons would be learnt" from the inquiry into the latest fatalities.
'In vain'
But Labour MP Paul Flynn said the UK's strategy was "wholly mistaken" as the Afghan police were "endemically corrupt" and "mercenary".
"We cannot succeed in Afghanistan and we must stop now sending our young men to die in vain," he told the BBC.
The opposition parties are also continuing to step up the pressure on the government over Afghanistan.
Lord Ashdown: "We have also to think about what are the consequences of failure or withdrawal"
The Lib Dems have called for an urgent debate on Afghanistan in Parliament, saying there are "serious questions" for ministers to answer about the safety of troops and overall strategy.
Commons leader Harriet Harman said the issue of Afghanistan was regularly discussed at prime minister's questions.
While there was no time for a full day's debate before the end of the current session of Parliament later this month, she said she would look at the scope for one soon in the new session.
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