US and German authorities are investigating a possible terrorist plot that prompted a security alert for US facilities in Germany last month, officials said.
A US official on Friday said there is intelligence that is "considered to be credible indeed", but nothing specific on the target or timing of any attack.
US television reported on Friday that the plot was at an advanced stage. But the government official stopped short of confirming those reports, saying only that it was "not something that was simply an idle plot".
The US embassy in Berlin said on April 20 that security was being increased at diplomatic posts and military installations and urged Americans in Germany to take steps to protect themselves.
"This is the threat that is still out there," said the US official on condition of anonymity.
"A lot of people in a lot of places are working hard to unravel" the threat, the official said.
In Berlin, an interior ministry spokesman said there was no immediate danger to US installations in Germany.
CNN, citing a senior US official, reported that the plot involved members of an Al Qaeda affiliate who were planning an attack using bombs and small arms against US citizens or interests. The plot has been evolving for several months, CNN said.
Broadcaster MSNBC reported that the intended target was believed to be US military installations in Germany.
The German interior ministry spokesman said the plot "concerns a known issue that prompted US officials to issue a warning to Americans in Germany several weeks ago".
Germany's federal prosecutors office, which handles terrorism cases, has opened an investigation in the case, the spokesman said. He declined to reveal details.
A prominent member of Germany's Social Democrats, the governing coalition's junior partner, criticised the US policy of issuing alerts about possible terrorist attacks.
"The US regularly issues such warnings, and quite often they turn out to be unfounded," said Dieter Wiefelspuetz, the party's internal affairs spokesman. "It is often impossible to verify such warnings. That is not good for the German security services."
US State Department spokesman Sean McCormack said the US has an obligation to act on credible information.
"We have a legal requirement, as well as moral requirement, to pass along our best advice to our public, so that they can take steps to protect themselves," McCormack said.
A US defence official said there was no intelligence to substantiate an earlier ABC News report that an attack was "imminent". The security measures taken at US military installations after the April 20 warning, including European Command in Stuttgart, has not been elevated.
"The US European Command has not received actual intelligence that would substantiate these reports and as a result our force protection measures remain in place," the defence official said.