Afghanistan's security forces face a $4bn (£2.5bn) funding shortfall after 2014 – when they are supposed to take over the main responsibility for fighting the insurgency – raising questions over whether the Kabul government will have the resources to keep the Taliban at bay, the Guardian has learned.

The Afghan army and police combined currently number about 308,000, and are due to reach their peak strength of 352,000 by the end of next year. The western strategy is for the Afghan national security forces (ANSF) to take on an increasing share in the fighting, allowing the US, Britain and their allies to withdraw all their troops from combat by 2014 and focus on training and counter-terrorist operations.

However, a 352,000-strong ANSF is estimated to cost $8bn and US officials have told their European counterparts that Washington is only prepared to foot $3bn of that bill after 2014. Other donors are expected to come up with another $1bn, enough to finance a force of only 220,000 troops.

Foreign and Afghan officials in Kabul agree such force would only be able to hold the line against the insurgents if there was a breakthrough in peace talks, or a collapse in the Taliban, both of which currently look unlikely.

"The Americans have told us that Congress is not prepared to give Afghanistan more military aid than Israel, which means no more than $3bn," a European official said.

The fear that international financial support for Afghanistan will evaporate in 2014 as the west struggles to escape from chronic recession and western capitals seek to put the conflict behind them, has alarmed the Kabul government of Hamid Karzai. Karzai will chair an international conference in Bonn on Monday designed to seek international commitments to continue to support Afghanistan long after 2014, when the country, after more than three decades of war, will continue to be an economic basket case. A World Bank report last week projected Afghanistan's fiscal deficit by 2021-22 at 25% of the country's GDP.

"Even assuming ambitious targets for robust growth in domestic revenue are met … there will be an unmanageable fiscal gap," the World Bank report said.

An agreement on how to fill the current ANSF financing gap will not be attempted until the Nato summit in Chicago this summer. Meanwhile, US officials have made it clear that other countries will have to come up with more than the $1bn they are currently projected to contribute to the ANSF and are warning the Afghan government that it may have to make do with a significantly smaller force over the long term than had originally been planned.

One senior US official said: "I don't think anybody would see the 352,000 figure as a permanent figure. It is the Afghan surge. I'm not aware of any plans not to reach it [next year], but it would be an open question how long it could be held at that level."

For Afghan officials, the question mark over continued financing brings back uncomfortable memories of 1989, when the Soviet army withdrew and western support for Afghanistan vanished, leaving the country to slide into prolonged civil war.

Abdul Rahim Wardak, the Afghan defence minister, said a dramatic shortfall in international support could lead to a return to chaos.

"Afghanistan has always been a proxy battleground in the past so if we are left alone and isolated as we were left in 1989, then the risk [of a repeat] is there, definitely," Wardak said in an interview in Kabul. "If Afghanistan is left by itself, it can become an ungoverned area or a failed state … It can become once again a place terrorists can hide they can take shelter, they can train and they can plan and they can operate from."

Wardak has been privately criticised by western officials for demanding tanks and fighter jets for the Afghan army, with limited application for the sort of counter-insurgency it is currently involved in, but he argued such heavy equipment is necessary psychologically, to send a message to the Afghan people and the country's enemies alike.

"My main purpose is that it will bring up the level of confidence of the Afghan people. They are a society which is used to seeing a lot of that stuff, and with anything less than that, they are always doubtful," the defence minister said. "So aircraft and tanks and heavy stuff will no only bring up the confidence of the people. And it will send a really strong message to our enemies that the odds [against them] are much higher."

Diplomats in Kabul said they saw the $3bn US figure as an opening offer in what it is likely to be a protracted negotiation leading up to the Nato summit in Chicago.

"The United States is keen to lessen the burden on themselves, there is less money around in general and the Americans won't commit to multi-year funding. That is going to mean a negotiation with the Afghans about the size of their army, and a negotiation among the allies on who is prepared to do what," a western diplomat in Kabul said.

In London, an official from the Department for International Development pointed out that the US and its allies were currently spending more than $100bn a year on the war, so full support of the ANSF would be a bargain, in relative terms.

"In the scheme of what is spent on international security at present, it is paltry. But the track record of the international community in providing long-term financing is not great," the official said. "If the international community can find the political will for many multiples of that [needed] figure now, it should be able to find the political will now … But failure to address this will lead straight back to conflict."