The US State Department says it has "credible indications" there is an imminent threat of attacks against "American interests".
While advising its citizens to exercise caution, the State Department said they had no information on specific targets, timing or method of attack but said suicide bombings were among their concerns.
It warned US citizens to be more conscious of their surroundings at places where Americans are generally known to congregate or visit, such as clubs, restaurants or places of worship, or to avoid those locations altogether.
"We remind American citizens to remain vigilant with regard to their personal security and to exercise caution," the statement advised, adding that "terrorist groups do not distinguish between official and civilian targets."
The warning supersedes a similar statement issued in March and will be in effect until October 1st.
The Department also urged Americans overseas to monitor the local news and maintain contact with the nearest American Embassy or Consulate.
The US government issued a "Worldwide Caution" on March 17th, warning of "credible reports that extremist individuals were planning additional terrorist actions against US interests".
The FBI has issued a warning to state and local law enforcement agencies to be on alert July 4th as Americans prepare to celebrate the Independence Day holiday.