Afghan President Ashraf Ghani on Sunday replaced two of the country's top security chiefs with staunch anti-Taliban officials, in a major shake-up days after US President Donald Trump's decision to slash troop numbers in the country.

Amrullah Saleh and Assadullah Khaled have been appointed to the critical posts of interior minister and defence minister, respectively, a presidential decree said.

Both are former Afghan intelligence chiefs who have blamed Pakistan for the Taliban's resurgence in recent years and have even called for it to be declared a state sponsor of terror.

There was no official explanation for the sudden reshuffle.

But it comes four months after Ghani rejected the resignations of former interior minister Wais Ahmad Barmak and defence minister Tariq Shah Bahrami following criticism over an increasingly deadly insurgency.

The move caps a tumultuous few days for Afghanistan after an American official told AFP late last week that Trump had decided to pull out “roughly half” of the 14,000 US forces in the country.

The unexpected move stunned and dismayed foreign diplomats and Afghan officials in Kabul who are intensifying a push to end the 17-year conflict with the Taliban.

While the Taliban has not issued a formal statement on Trump's plan, a senior commander told AFP the group was “more than happy”.

Trump's decision apparently came on Tuesday as US peace envoy Zalmay Khalilzad met with the Taliban in Abu Dhabi, part of efforts to bring the militants to the negotiating table with Kabul.

Observers fear the hasty move could undermine Khalilzad's negotiating position, embolden the Taliban, and further erode morale among Afghan forces, which are being slaughtered at a record rate.

Ghani, who is planning to seek re-election in April, could be trying to strengthen his security credentials ahead of the vote.

Saleh, a fierce government critic who fought against the Taliban in the 1990s, served as head of the National Directorate of Security (NDS) from 2004 to 2010.

Khaled was briefly NDS chief in 2012 before being wounded by a Taliban suicide bomber.

Both men will serve as acting ministers until the parliament approves their appointments.