Brown tells British troops they are the 'country's true heroes' during surprise Afghanistan visit

On a flying visit to the Army's main base in Afghanistan, the Prime Minister said their achievements in the war zone surpassed those of Britain's medal-winning Olympians.

But the Premier, who also pledged an extra £120million to help rebuild the battle-scarred country, failed to shake off questions over his domestic troubles.

Gordon Brown speaks to troops at Camp Bastion in Helmand Province, Afghanistan.

During a joint press conference in Kabul with President Karzai he faced repeated questions over whether his own position was under threat from Foreign Secretary David Miliband. 

And then he had to suffer the embarrassment of Mr Karzai having a joke at his expense.

A clearly-irritated Mr Brown insisted he was concentrating on Britain's economic difficulties and that he had a 'good relationship' with Mr Miliband.

He said: 'We get on with the job. What people want us to do is get on with the economic problems that face our country and all countries.

'I am getting on with the job and that is what people expect us to do.'

But Mr Karzai, who faces numerous challenges to his own leadership, interjected  as reporters pressed the British Prime Minister over his relationship with David Miliband.

'Cabinet ministers plotting is nothing new. We have it in Afghanistan - although not my foreign minister,' Mr Karzai remarked.

Mr Brown looked irritated by the intervention and the persistent questions, repeatedly insisted:  'It is a good relationship. We get on with the job.'

Meanwhile, in a fresh sign of insurgency violence, three Nato soldiers died in a roadside booby-trap.

It was also reported that an American journalist who had been investigating terrorists had gone missing in Pakistan -- carrying a chilling echo of Daniel Pearl, the writer who was beheaded after being kidnapped by militants in 2002.

Nicholas Shamble, who edits US magazine Smithsonian, vanished two days ago. He was at a hotel in Karachi on August 12 and left for Islamabad on Monday but never arrived.

Relaxed: Gordon Brown and wife Sarah chats to reporters on his plane to Beijing last night

Mr Brown made a 90-minute stopover in the searing heat of Camp Bastion, where UK forces are based, en route to the Olympic games in Beijing.

He told them they had displayed 'exactly the same courage, professionalism and dedication' as the competitors in Beijing.

In a speech to some 300 troops from 16 Air Assault Brigade, he said the nation owed them a 'huge debt of gratitude' for fighting the Taliban.

He said: 'This week we are celebrating the Olympics where we have had great success.

'But this week also I believe that our Olympic athletes and everybody else in our country will remember that you have showed exactly the same courage, professionalism and dedication.

'You make our country proud every day of the week and every week of the year. You are truly the heroes of our country.'

He added: 'You know that you are in the frontline in the fight against the Taliban. You know that by what you are doing here you prevent terrorism coming to the streets of Britain.'

He also paid tribute to Cpl Barry Dempsey, the latest British soldier to die, and visited a field hospital where he spoke to six injured soldiers.

Mr Brown said the Taliban had switched to guerrilla-style tactics, such as suicide bombs and roadside explosions, rather than 'head on confrontation' with troops.

Since the US-led invasion in 2001, 116 British troops have died in Afghanistan.

The dangers faced by the British in the badlands of southern Afghanistan were underlined by the death this week of 10 French troops in a Taliban ambush just 30 miles from Kabul.

Today three Polish soldiers serving with NATO forces died in a roadside blast in the central province of Ghazni.

It added weight to warnings from Western commanders that insurgency attacks were growing larger and more sophisticated.

After holding talks in Kabul with Afghan President Hamid Karzai, Mr Brown pledged to inject more resources into Afghanistan.

The Prime Minister wants to see the size of the 60,000-strong Afghan army doubled to about 120,000 and he announced an extra £64million to pay teachers' salaries and fund development work, especially healthcare.

He said work would continue to ensure Afghan police were 'corruption free' and to improve local and national government administration by developing a civil service in Afghanistan. More money would also be put into improving education.