President Barack Obama's decision to order a bigger and speedier reduction in troop numbers than advised by his top military commanders will increase American casualties during the pull-out, Pentagon planners fear. "The fighting will be intense as they leave and the planners expect the casualty rate will increase even as troop numbers are being reduced," a senior US defence department adviser told The Sunday Telegraph. "That scares the cr-p out of people at the Pentagon." Planners at the Joint Chiefs of Staff also believe that the announcement last week that a third of the 100,000 US personnel will be brought home by next autumn will lead to accelerated withdrawals by Nato allies a trend that would in turn increase public pressure in the US for even more US forces to leave. Those concerns of a "vicious circle" momentum of drawdowns, driven by and then further contributing to public pressure in the US and Europe for withdrawals, were fuelled when France immediately said that it would being a phased pullback of its 4,000 troops. David Cameron, who has pledged that there will be no British combat forces in Afghanistan by 2015, welcomed Mr Obama's speech. "Where conditions allow, it is right that we bring troops home sooner," the prime minister said. On the ground, US commanders believe the announcement means it will now be necessary to abandon plans to take their fight against the Taliban to the violent eastern mountains bordering Pakistan where the Afghan war was born. They had wanted to try and repeat the successes since Mr Obama's "surge" was begun in 2009 during fierce fighting in the southern provinces of Kandahar and Helmand. A senior Pentagon adviser who attended discussions with planners at the Joint Chiefs of Staff, the country's most senior military body, revealed the scale of concerns inside the US defence department. Admiral Mike Mullen, head of the Joint Chiefs, and Gen David Petraeus, commander of Nato forces in Afghanistan whom Mr Obama has nominated as his next CIA director, both made clear last week that they had opposed the president's plans. But the true extent of military planners' fears can now be disclosed following the private meeting to discuss the president's announcement that 10,000 troops would be withdrawn this year, followed by next summer by the remaining 20,000 personnel from his 2009 "surge". The simple logistics of arranging such a large and rapid withdrawal from a country with such basic infrastructure and extremely limited and exposed transit routes is one of their greatest fears. "They expect casualty rates will increase as a result of this order to get so many troops out so quickly," the adviser said. He said that they were also worried that Mr Obama's declaration that the US had largely achieved its goals in Afghanistan will undermine already-weak public support for the US mission there and provide cover to other Nato countries that want to cut their operations there. "They believe that this could set off a rush to retreat and pull the plug on the whole mission. They are worried that they will be caught in a nightmare of a collapsing military campaign in Afghanistan and a collapse in backing at home." He said they feared the development of a "Dien Bien Phu mentality" referring to the scene of France's humiliating battle defeat to communist fighters in Vietnam that led to a cave-in of French support for their presence in Indo-China. US commanders also fear that they could squander gains made against Taliban strongholds in the south. And they accept that they will not have the manpower to launch similar offences in the east against Taliban forces there and the notorious Haqqani network run by an Afghan warlord and his sons. "The planners believe that their best hope now is to focus on the south and consolidate what they've got," the adviser said. "In the east, you will see an escalation of drone attacks and perhaps commando raids. But they had wanted to put boots in on the ground in the sorts of numbers that they have done in the south. That doesn't seem practical now. "Remember, the military had asked for 50,000 extra men to get the job done. In 2009. Obama gave them 30,000. And now he's going to start to pull them out before they have completed two fighting seasons." John Pike, a prominent US defence analyst and director of GlobalSecurity.org, criticised Mr Obama's address to the nation as "pathetic". "He campaigned on Afghanistan as 'a war of necessity', but on Wednesday he completely failed to articulate why we are there. It seems to me that he had an obligation to the troops and to the country to explain what we're doing there. But he didn't even try. "That lack of articulation reflects the lack of strategy. He didn't explain his strategy as he doesn't have one. We're just muddling through. "I think he just goes into the room with his advisers, hears everybody out and then makes his decision by splitting the difference. "Well, splitting the difference might be OK if you are working on a budget deal, but it's not a war-fighting strategy."