Katherine Jane Wilson, a well-known Australian NGO worker, was kidnapped April 28 in the city of Jalalabad, close to the border with Pakistan.
"The US Embassy Kabul reminds US citizens that the threat of kidnapping and hostage taking continues to be very high," the diplomatic mission said in a statement reported by AFP on Friday.
"The US Embassy continues to take this threat seriously and advises... US citizens to take appropriate security precautions and to avoid predictable travel patterns within Afghanistan."
The statement added that an attempted kidnapping last Monday targeted several expatriates, including a US citizen. It offered no further details.
Australian Foreign Minister Julie Bishop has said that Canberra is working to secure the release of Wilson, but insisted Australia does not pay ransoms for hostages.
Wilson, said to be aged 60, ran an organization known as Zardozi, which promotes the work of Afghan artisans, particularly women.
The abduction has set off alarm bells among foreign residents in Afghanistan.
Aid workers in particular have increasingly been casualties of a surge in militant violence in recent years.
In April last year the bullet-riddled bodies of five Afghan workers for Save the Children were found after they were abducted by gunmen in the strife-torn southern province of Uruzgan.