The State Department called on US citizens in the capital, Sanaa, on its website on Monday to take exceptional security measures.
The embassy has also invited prominent business people to a security meeting to define the possible dangers.
State Department spokesman Richard Boucher declined to discuss specifics, but a British diplomat suggested the Sanaa Trade Centre and US-affiliated restaurants and shops in the Hadda area were possible targets.
US citizens have been urged to avoid all non-essential travel to Yemen for many months.
The new alert was issued as Yemen begins a programme to curb Islamist attacks by replacing more radical preachers at certain mosques.
The country is also embarking on the second stage of a security plan that involves setting up new police and traffic outposts on major roads.
The alert also followed the State Department's renewal on Friday of an existing "worldwide caution" warning US citizens about the ongoing global war with al-Qaida and specifically noted the "potential for threats to maritime interests."
Boucher would not elaborate, but
an attack seems highly likely
Yemen has been the site of two sea-borne attacks attributed to al-Qaida followers since 2000 and the anniversaries of both are fast approaching.
"It's an ongoing threat and the fact that these anniversaries are coming up ... is part of that ongoing threat," Boucher said.
On 12 October 2000, al-Qaida were suspected of carrying out a suicide bombing against the USS Cole destroyer in Aden harbour, killing 17 US sailors and wounding 38 others.
On 6 October 2002, nearly two years to the day after the Cole bombing, suspected al-Qaida operatives hit the French supertanker Limburg as it prepared to enter al-Shir port on Yemen's southeastern coast, killing one and wounding 12 others.