BERLIN, Germany (CNN) -- German police arrested three suspected Islamic militants who were planning "massive" and "imminent" attacks on American targets in Germany, authorities said Wednesday.

"The main motivation of the group in Germany is hatred against American citizens, and therefore they had as main targets the American military installations," said Jorg Ziercke, president of Germany's Federal Criminal Investigation Office.

"This could also of course have affected German citizens in restaurants and other places."

A counterterrorism source in Frankfurt with knowledge of the plot told CNN a sophisticated detonator -- of the type that can be used in a military device -- was found in the possession of some of the suspects. This type of detonator is difficult to get, the source said. It is more precise and can inflict more casualties than lower-grade detonators.

Investigators are trying to determine how the suspects obtained it, the source said.

Authorities would not elaborate on whether Ramstein Air Base -- the U.S. military's main installation in Germany -- or the major international hub of Frankfurt Airport were among the targets, as reported by German media.

"There are, of course, strong grounds to believe that it could also have been directed at American military installations ... it also could also have been Frankfurt Airport, we can't really rule that out," German Interior Minister Wolfgang Schaeuble said.

Ziercke said the men are members of a German terror cell called the Islamic Jihad Union and received terrorist training in Pakistan.

The Islamic Jihad Union is an offshoot of the Islamic Movement of Uzbekistan -- an affiliate of al Qaeda and a signatory to a 1998 fatwa from Osama bin Laden in which he declares holy war on the West and Israel, according to the counterterrorism source. The group, which has links to al Qaeda, was founded in December 2006, he said.

All three had been under surveillance for more than six months, a spokesman for Germany's Interior Ministry told CNN.

One of the two Germans was arrested in December for spying on a U.S. military installation in Hanau, Germany, he said.

German media identified the two Germans -- both converts to Islam -- as Fritz G., 28, and Daniel S. 22; the Turkish man was identified as Adem Y., 29.

Schaeuble said the radical Islamist center was under surveillance and authorities "knew this center in Ulm was playing a role" in the terror plot, although he did not elaborate.

The suspects were believed to have been planning car bomb attacks against discos, pubs, airports and other places frequented by Americans, German federal prosecutor Monika Harms said.

"These weren't just sort of half-professional terrorists -- these were very dangerous, highly professional men," Schaeuble said.

"We were able to succeed in recognizing and preventing the most serious and massive bombings," Harms told reporters.

Harms said the group had amassed 680 kilograms (1,500 pounds) of hydrogen peroxide -- the same chemical used in the London transport bombs that killed 52 people and four terrorists in 2005.

"This would have enabled them to make bombs with more explosive power than the ones used in the London and Madrid bombings," Ziercke said at a joint news conference with Harms.

"Detonators and electronic components" were also found during a recent raid on a holiday house in Germany's Black Forest region.

"Yesterday they began to manufacture the explosive materials," she said. "They were close to completing the manufacture of this material, and the criminal office managed to arrest them before they completed the task."

But Schaeuble pointed out that the men "did not get to a very dangerous stage because happily, at an early stage, they attracted the attention of our officials."

He said "potential accomplices" that had already been picked up by authorities led to Tuesday's arrests.

The men are suspected of having links to an Islamic group abroad -- which had split from a group in Uzbekistan -- and a German group of the same name, authorities said.

The United States is "closely monitoring" the terror arrests in Germany, as well as Tuesday's arrests in Denmark, Department of Homeland Security spokesman Russ Knocke told CNN.

"At this time, there is no credible information telling us of an imminent threat to the homeland, but we do believe that we continue to be in a period of increased risk," he said.

Schaeuble said he had "no knowledge of any link" with the Danish arrests of eight suspected Islamic militants accused of storing explosives in a populated area of Copenhagen with the intent of carrying out a terror attack.

But the German interior minister added there was "a strong parallel" between the two investigations and subsequent arrests.

President Bush -- who is in Sydney, Australia, ahead of the annual APEC summit -- was briefed on the arrests at his daily intelligence briefing Wednesday morning, National Security Council spokesman Gordon Johndroe said.

"He's pleased a potential attack was thwarted and appreciates the work of the German authorities and the cooperation by international law enforcement," he said.

"The arrests today remind of us the threat that terrorists pose around the world and the need to continue to pursue them wherever they are."

A U.S. government official who did not want to be named called the German terror plot "the real deal," adding that U.S. authorities "have been working this case real hard."

The arrests in Germany are linked to a threat first reported several months ago, sources said.

The United States has been working with the Germans since the "earliest stages of the investigation" which has been tracked by the Germans for some time, according to a U.S. counterterrorism official.

The FBI will be seeing if there are any U.S. connections to these men, the U.S. government official said.

When asked about whether the United States was aware of the alleged German plots, Knocke said, "We have very close ties with our German counterparts" but did not elaborate on specific plots.

"We have met repeatedly with our German counterparts on a variety of security measures and consider them to be among our most important allies," the DHS spokesman said.

German authorities had alerted the U.S. European Command in Stuttgart of a possible terrorist threat to American installations, but not specifically Ramstein, Capt. Jeff Gradec said. Neither EUCOM nor Ramstein is taking any extra security measures, the U.S. military said.

Ramstein Air Base is in western Germany's Rhineland-Palatinate state and is about 125 kilometers (75 miles) southeast of Frankfurt.

Wolfgang Bosbach, a top legislator for German Chancellor Angela Merkel's Christian Democrats, said the suspects had been under observation by security officials for a long time."

Bosbach told N24 television an attack could have occurred "in a few days," possibly to coincide with the anniversary of the September 11, 2001 attacks in the United States and an upcoming German parliamentary debate on troop levels in Afghanistan.

"We are in a highly sensitive period," he added.

CNN international security correspondent Paula Newton said intelligence officials have been calling for more cooperation to combat terror plots in Europe, in particular the faster transfer of information among different countries.

Europe is at high risk, officials say, due not only to the Iraq war, but also the NATO mission in Afghanistan, to which many European countries contribute, she said, adding that the Muslim population in Europe is beginning to feel more alienated than it has in previous decades. "This brings Europe to the battleground," Newton said