The Afghan Taliban in an "open letter" to United States (US) President Donald Trump reiterated their calls for America to leave Afghanistan after 16 years of war.

In a note in English that was sent to journalists on Tuesday by Taliban spokesman Zabihullah Mujahid, the insurgents said that Trump had recognised the errors of his predecessors by seeking a review of the US strategy for Afghanistan.

Mujahid said Trump should not hand control of the US Afghan policy to the military but rather, announce the withdrawal of US forces and not an increase in troops as the Trump administration has planned.

The note, which is 1,600 words long, also says a US withdrawal would "truly deliver American troops from harm's way" and bring about "an end to an inherited war."

US Defence Secretary James Mattis in July said that the Trump administration’s new strategy for Afghanistan would have a regional context, including a Pakistan angle.

He said that while media speculations about the Trump administration sending close to 5,000 additional troops to Afghanistan "may turn out to be right", the new strategy "also involves, perhaps, changing somewhat what the troops on the ground are doing right now".

Some US officials have questioned the benefit of sending more troops to Afghanistan because any politically palatable number would not be enough to turn the tide, much less create stability and security.

To date, more than 2,300 Americans have been killed and more than 17,000 wounded since the war began in 2001.

Any increase of several thousand troops would leave American forces in Afghanistan well below their 2011 peak of more than 100,000 troops.

The Afghan government was assessed by the US military to control or influence just 59.7 per cent of Afghanistan's 407 districts as of Feb 20, a nearly 11 percentage-point decrease from the same time in 2016, according to data released by the US Special Inspector General for Afghanistan Reconstruction.