Acting Pentagon chief Patrick Shanahan says there is no order to withdraw US forces from Afghanistan and the Afghan government should participate in peace talks with the Taliban, in remarks on a surprise trip to Afghanistan on Monday.

Shanahan met the US commander and troops and Afghan President Ashraf Ghani in the unannounced one-day visit, his first in his new role.

Reports that Donald Trump, president of the United States wants to withdraw about half of the estimated 14,000 US forces in Afghanistan, has raised concerns among Afghan and regional officials about the effect it might have on security in the country.