German security services foiled a plan by Islamist militants to carry out "massive bomb attacks" against U.S. installations in Germany and arrested the three men behind it, officials said on Wednesday.

Federal prosecutor Monika Harms said the men, two German nationals and one Turk, had been on the verge of launching their attacks after acquiring enough material to make a bomb with explosive power equal to 550 kilograms of TNT.

The men were arrested on Tuesday, the same day Danish police seized eight young Muslims they suspect of plotting a bomb attack and one week before the sixth anniversary of the Sept. 11, 2001 hijacked plane attacks on targets in the United States.

"Thanks to the cooperation of federal and local police over several months, we were able to ... prevent massive bomb attacks," Harms told a news conference in Karlsruhe.

German Defence Minister Franz Josef Jung described the security threat posed by the suspects as "imminent", while Chancellor Angela Merkel said a "horrible event" had been averted.

"It shows that the terrorist threat here isn't abstract. It's real," she said at a news conference in Berlin with visiting Lebanese Prime Minister Fouad Siniora.

Officials could not confirm reports the accused had been targeting Frankfurt international airport and a major U.S. military base in Ramstein, but said they had been seen scouting out U.S. installations such as discos, pubs and airports.

"The apparent motive is hatred of Americans," said Federal Police chief Joerg Ziercke, adding that the explosives could have caused more damage than bombings in Madrid and London which killed 191 and 52 people.

The three belonged to a little known al Qaeda-affiliated Sunni Muslim group with roots in Uzbekistan called "Islamic Jihad Union", officials said. All three are believed to have trained in Pakistan militant camps.

It was unclear whether there was any link between the alleged German and Danish plots, official said.

Germany, which has forces stationed in Afghanistan, has been on high alert for attacks. The country has feared a re-emergence of militant Islamist groups since 2001, when the northern city of Hamburg was used as a base for planning the Sept. 11 attacks.

In April the U.S. embassy in Berlin announced it was boosting security at diplomatic and military facilities in Germany in response to an increased threat of terrorism there.


RAID ON HOUSE

Ziercke said the men had been seized at a rented holiday house in the Sauerland region of North Rhine-Westphalia in western Germany.

Between February and August 2007, he said, the accused had acquired 12 large vats and filled them with a 35 percent hydrogen peroxide solution totalling some 730 kilogrammes.

"This amount would have been enough to cause damage on a greater scale than in London and Madrid," Ziercke said. He said the suspects were likely planning simultaneous car bomb attacks in locations across Germany.

Some 300 police had been tracking the suspects since December, when one of them was observed scouting out U.S. military facilities in the German city of Hanau, near Frankfurt.

The police swooped on Tuesday, raiding the house and some 40 other sites across Germany after local police unwittingly stopped the suspects on a routine traffic violation several days ago and they grew nervous, Ziercke said.

One suspect escaped out of a bathroom window but was detained after a scuffle with police in which a shot was fired. Police said additional arrests were possible.

"The United States congratulates the German government on its success in breaking up the plot by terrorists to attack targets in Germany," a U.S. embassy official in Berlin said.

Germany has not seen a major attack in several years. But concerns about an attack have mounted since two men of Lebanese origin tried to detonate crude bombs hidden in suitcases on trains last year. Prosecutors have said those bombs failed to go off because of a technical flaw.

"We are under threat," Interior Minister Wolfgang Schaeuble told a news conference in Berlin. "We have to remain vigilant."