Afghanistan's air force is already woefully short of planes and skilled pilots -- and US plans to scale back supportive air strikes will leave it without crucial back-up as it battles a resilient Taliban.

For the past 13 years a vast fleet of US fighter jets, attack helicopters, unmanned drones and transport aircraft have supported ground troops in operations against the insurgency.

But with the vast majority of the 51,000 foreign troops set to pull out by December, the fledgling Afghan Air Force (AAF) will have to make do without much of this help.

US officers say there is little doubt that air strikes by Apache helicopters or American warplanes will be less frequent from 2015, and that the conditions that trigger a raid will be stricter.

A US contingent of 32,000 is due to drop to 9,800 by 2015, and most will be based at two hubs in Kandahar and Bagram airfields. That will leave few troops available to call in air strikes from the ground.

While the Afghan army and police have made strides on the battlefield, airpower remains a major weakness, and the Afghans often rely on US aircraft when faced with ambushes or entrenched Taliban fighters.

"Afghanistan has a significant need for air support, but the AAF cannot support more air power than is currently planned," US think-tank CNA said in a study released earlier this year.

"The AAF is struggling to find sufficient numbers of qualified recruits to grow to its planned size. Even if additional recruits are found, only a small number could be fully trained by 2018."

Airpower is crucial in rugged Afghanistan, where the poor road network is often mined by insurgents. The Afghan government has long pleaded with international forces to give them advanced aircraft before the troops withdraw.

- Building an air force -

The AAF now has 88 aircraft -- including Cessna-208 light transport and surveillance planes, and Cessna 182 Turbo Skylane aircraft -- as well as nine Mi-17 helicopters, according to the US watchdog agency Special Investigator for Afghanistan Reconstruction.

In the long run, Afghanistan hopes to build up its own airpower with 20 Super Tucano A-29 aircraft -- turboprop planes that the United States is buying for Kabul.

The aircraft look crude compared to sophisticated fighter jets, but officials say they are perfectly suited to Afghanistan and will provide an effective means of striking at insurgents from the air.

The Tucanos, which are inexpensive to operate, have machine guns mounted on the wings and can carry laser-guided bombs.

But their delivery has been delayed for years by legal disputes over the contract -- the work went to Brazilian aerospace firm Embraer, and US rival Beechcraft protested.

Beechcraft's objections were rejected in the end, but it will take time before a corps of pilots is ready to fly the A-29s.

"It's a topic that we discuss often," said US Army Major Phil Martin, an adviser to Afghan forces in southern Kandahar province.

"They want us to provide as much support to the end, which is totally understandable."

The US wants to provide that support, he added -- but "at the same time, we want to develop their systems to the fullest capability".

- Unfinished pact -

The Afghans remain reluctant to strike the Taliban with mortars and artillery as they are worried about causing civilian casualties, Martin said.

"They are very cognizant of collateral damage. They know that mortars and artillery are going to destroy buildings."

US airpower offers "precision" and a more effective alternative, he added.

Its role in aiding Afghan forces after 2014 will be hammered out once a long-delayed bilateral security agreement (BSA) between Washington and Kabul is signed, according to officials.

"No one knows the answer to that right now," said a senior US official. "It will be worked out in the BSA."

Outgoing President Hamid Karzai has refused to sign the BSA, but both presidential candidates vying to succeed him in next week's run-off election have promised to ink the deal, which lays out the legal framework for US forces stationed in Afghanistan after this year.

In any case, air strikes overall in Afghanistan have decreased in recent months as international troops pull back from combat.

American aircraft carried out 21,785 close air support missions last year in Afghanistan for both the international and Afghan forces, according to the US Air Force. So far this year, there have been more than 6,000 such sorties.