Not a single Afghan police or army unit is capable of maintaining law and order in the war-torn country without the support of coalition forces, The Independent on Sunday can reveal. Almost a decade after international troops were sent in to overthrow the Taliban and help to establish a functioning democracy in Afghanistan, a combination of poor training, lack of numbers, corruption and illiteracy has left the country unable to protect its own people.

The grim official assessment of the capabilities of the Afghan National Security Forces (ANSF) is a major blow to the hopes of a troop withdrawal by 2014, a timescale that assumes the ANSF will be able to start taking the lead in fighting the Taliban from next month. The commander of Nato's mission to train the ANSF has admitted the task will not be complete until at least 2016.

This comes after a decade in which tens of billions of dollars have been spent building up the Afghan army and police. Yet they remain too dependent on coalition forces, according to the latest progress report on Afghanistan from the US Department of Defense. It cites assessments made in February that show how, of more than 400 Afghan units, none is rated as independent – defined as capable "without assistance from coalition forces".

The International Security Assistance Force (Isaf) will release another assessment this week, but experts are not expecting any major change. There are some signs of progress, with army units deemed effective "with advisers" up 45 per cent (from 42 to 61) and those effective "with assistance" rising by 73 per cent (from 49 to 85) between September 2010 and February 2011. But more than half of all police and army units need coalition soldiers to fight alongside them. Only a third are effective with just military advisers in support.

The report, looking at the period between October 2010 and March 2011, warns: "ANA [Afghan National Army] units are still too dependent on coalition forces for operations and, specifically, logistical support."

A "significant shortfall" of 740 trainers and mentors "poses a strategic risk to ANSF growth and an increased risk to transition". Citing the "slow development of governance", it warns that "despite the gains in security and in ANSF growth and development, the Afghan government faces significant political challenges which could potentially threaten the progress made in the last six months" while "extensive bureaucracy and areas of corruption continue to present serious challenges".

The US has spent more than $25bn (£15.4bn) to train and equip Afghan forces since 2001. The Afghan army has almost doubled in size in three years to more than 164,000, and the Afghan national police stands at 126,000. But the effort has been marred by focusing on quantity over quality, exacerbated by corruption, a continued shortage of trainers and infiltration by the Taliban.

Literacy remains a problem. Although more than 63,000 ANSF have completed literacy training since November 2009, none is above the reading age of a nine-year-old. Lieutenant General William B Caldwell, commander of the Nato's training mission in Afghanistan, admitted, "Nine out of 10 [new recruits] are totally illiterate and innumerate... Attrition in the army, if left unchecked, could undo much of the progress made."

Speaking in Washington last week, he hinted that politically driven timescales do not match reality. "Building quality into the ANSF requires strategic patience and enduring commitment... As far as the training mission goes, we won't complete what we need to do till about 2016/17."

This admission comes as coalition forces prepare for another bloody summer. Nato-led troops suffered record losses in April and May with 110 dead, the highest death toll for those two months since the war began. The Taliban spring/summer offensive is under way, marked by a string of high-profile assassinations of Afghan officials in recent weeks.

Bomb attacks across Afghanistan killed at least 21 people yesterday, as a UN report said May was the deadliest month for Afghan civilians since it began compiling statistics four years ago, with 961 killed or injured.

Yet President Barack Obama is due to start drawing down US troops this summer. And if he uses the death of Osama bin Laden to justify a faster exit than expected, this could trigger a larger withdrawal of British troops. However, the US defence secretary, Robert Gates, insisted last week there would be no "rush to the exits". Initially, Bamiyan and Panjshir, anti-Taliban strongholds, are the only provinces that will be handed over. Afghan forces will also take responsibility for the western city of Herat, the northern city of Mazar-i-Sharif and Lashkar Gah in Helmand.

Ahmad Majidyar, senior research associate at the American Enterprise Institute, said: "Lack of equipment remains ANA officers' primary concern, who say the army was better prepared under the Soviets than now."

What the experts say... 'The real issue is corruption'

"They face serious challenges developing capacity, and growing problems in maintaining loyalty, morale and troop cohesion as the insurgency gains ground andtalk of deals with the Taliban divides Afghan people alongethnic lines."

Candace Rondeaux; Senior analyst, International Crisis Group

"How many Afghan generals, lieutenants, and officers have you seen falling in battles over the past 10 years? We lose soldiers but not officers. What would motivate those soldiers to defend a territory against the Taliban?"

Yama Torabi; Director, Integrity Watch Afghanistan

"The ANSF can still lead while receiving logistics support. They are the pointy end of the spear. We'll still make up some of the shaft of the spear for a while yet."

Dr Paul Miller; Former director for Afghanistan on the US National Security Council

"The real issue is corruption – to what extent is the ANSF insulated from that? If not, we could spend money on training and equipment for a million years and it wouldn't produce real improvement."

Dr Stephen Biddle; Senior fellow for defence policy, Council on Foreign Relations

"Most of their British trainers say that the quality of ANA units is 'OK', and that they are competent to do the basic job."

Professor Michael Clarke; Director, Royal United Services Institute for Defence and Security Studies

"The Western-led ANSF build-up is rather quantitative than qualitative, with no guarantee of sustainability. Who will pay for all these soldiers, police and militiamen when Western financial inputs decrease after 2014?"

Thomas Ruttig; Co-director, Afghan Analysts Network

"We did not seriously invest in this force until 2008; the money did not really show up, along with the equipment and the facilities, until the spring of last year."

Anthony Cordesman; Chair in strategy, Center for Strategic and International Studies

"While the ANSF may be able to take increasing responsibility alone in some of the lower-risk areas, they're nowhere near ready to do so in the most troubled provinces, such as Kandahar and Helmand."

Colonel Richard Kemp; Former commander of British forces in Afghanistan

"Ethnic and tribal factionalism and weak civilian oversight risk ANA's disintegration if Nato forces leave prematurely, as happened after the Soviet 1979 withdrawal."

Ahmad Majidyar; Senior research associate at the American Enterprise Institute

"They are entering into a critical period of development, and it's a real time of uncertainty. We realise they're going to be tested very heavily by the enemy."