NATO forces are struggling to build an Afghan army big enough to take over security responsibilities in the country.

This is because of high desertion rates, a top US commander has revealed.

Training a credible Afghan security force, both army and police, lies at the heart of NATO’s exit strategy from a war that has already lasted nine years and President Barack Obama’s hopes of beginning a drawdown of US troops from July next year.

Despite intense training and literacy lessons, 23 per cent of Afghan army recruits taken on to serve with coalition forces against the Taliban, and 14 per cent of the police, are leaving their posts and vanishing, according to Lieutenant-General William Caldwell, the US commanding-general of NATO’s training mission in Afghanistan.

Underlining the challenge he faces to increase the size of the Afghan National Security Force to 305,000 by the end of October next year, General Caldwell said that to produce the next 56,000 trained Afghans he was going to have to recruit 141,000 just to meet the target. That is a number equivalent to the current size of the whole Afghan army.

The “attrition rate”, as General Caldwell called it, is one of the biggest obstacles facing US and other NATO trainers as they try to develop local security forces. General Caldwell said the desertion rate was considered to be “the endemic enemy”, undermining the efforts to meet Mr Obama’s timeline for bringing the first US troops home.

Although General Caldwell said significant progress had been made, especially in the past nine months, he explained that some crucial aspects of the program to convert the Afghan military into a proper professional organisation had still hardly started.

Afghan combat troops being mentored by the coalition forces could not survive on their own because they did not have any of their own back-up units trained

in intelligence, communications, transport and logistics. These key support elements were still all provided by the coalition.

Aware of the gaps that still exist, General David Petraeus, the overall US commander in Afghanistan, appeared to back-track on the testimony he gave to congress in July when he stated that he supported Mr Obama’s strategy to start pulling out US troops in July next year.

He told the BBC that if he felt the deadline for the first withdrawals was too risky, he would say so to the President.

General Caldwell’s warnings about desertions among Afghan soldiers and police officers highlight grave concerns among NATO trainers about their ability to persuade well-trained locals to stay in the job.

President Hamid Karzai, the Afghan leader, said last week that he believed that some of the deserters were being employed by private security companies, which he wants to evict from Afghanistan.

General Caldwell said that might be true. “The Afghans coming out of the 17-week training program are a whole lot (more employable) than they were when they went in,” he said.

To meet the challenge of high desertions, General Caldwell said Afghan soldiers and police were now better paid — $US165 ($186) a month –and offered bonuses.

General Caldwell admitted that until November last year, the training program for the Afghan National Security Force had been inadequate. There had also been no focus on the high illiteracy rate among the Afghans recruited.

“You can’t expect a soldier to account for his weapon if he can’t even read the serial number on his rifle,” General Caldwell said.