US officials have confirmed the change from code "yellow" to "orange" - the second most serious level, fearing an attack during the Christmas holiday period.
The announcement follows reports in the American media that al-Qaida is planning a new attack on a US target.
Homeland Security Secretary Tom Ridge said that any new strike could "rival or exceed" the September 11 attacks on the World Trade Center and the Pentagon in 2001.
"The strategic indicators including al-Qaida's continued desire to carry out attacks against our homeland are perhaps greater now than at any point since Sept 11, 2001," Ridge told reporters.
At the state department a "Worldwide Caution" issued on 21 November was renewed.
"The US government remains deeply concerned about the security of US citizens overseas," said a state department statement. "We are seeing increasing indications that al-Qaida is preparing to strike US interests abroad."
The increased threat comes two days after Aljazeera broadcast an audiotaped message from al-Qaida's second in command Ayman al-Zawahri.
In the message, Zawahri called American soldiers cowards "with no faith in their leaders". The speaker vowed to defeat the "infidel" United States and said al-Qaida would target Americans everywhere, including the United States.
The CIA has confirmed that the voice purporting to be the al-Qaida deputy was "most likely" al-Zawahri.
US Alert Warnings:
All-Zawahri warned that the group was pursuing Americans everywhere, but especially "in Afghanistan, in Iraq, in Palestine and in the Arab peninsula".
In an apparent reference to an attack on a Baghdad hotel in October, al-Zawahri said: "Americans have become unable to defend themselves and even to protect their biggest criminals and leaders such as (Deputy Defense Secretary Paul) Wolfowitz."
ABC television reported on Friday that US intelligence reports had suggested an increase in terrorist threats to a number of US cities.
The five-level colour-coded alert system has been at yellow since 30 May. The coded warning was upgraded to orange on 5 May. A red warning has never been issued.