The United States on Tuesday ordered non-essential staff to leave its Kabul embassy, citing increased threats as Washington prepares to end its 20-year war.

The State Department in a travel advisory said it had "ordered the departure from US embassy Kabul of US government employees whose functions can be performed elsewhere."

Ross Wilson, the acting US ambassador in Kabul, said that the State Department took the decision "in light of increasing violence and threat reports in Kabul." 

He said that the order affected an unspecified "relatively small number" of employees and that the embassy would remain operating.

"Personnel who are urgently needed to address issues related to the drawdown of US forces and the vital work we are doing in support of the Afghan people will be able to remain in place," Wilson wrote on Twitter. 

President Joe Biden earlier this month said he would withdraw all troops from Afghanistan by September 11, the 20th anniversary of the attacks that led the United States to invade and topple the Taliban regime which had welcomed Al-Qaeda. 

Biden concluded that US forces had achieved their objectives and could do little more, but US officials have not made any secret of fears that violence will rise as the Taliban perceives that they achieved victory. 

Under an accord reached by former president Donald Trump's administration, the Taliban agreed not to attack US troops as they withdraw but they have stepped up attacks on Afghan government forces. 

The State Department advisory, which also renewed warnings for Americans not to visit, said that "terrorist and insurgent groups continue planning and executing attacks in Afghanistan." 

The Biden administration will maintain limited forces in Kabul to guard the sprawling embassy.