BERLIN – Three suspected Islamic militants were arrested for allegedly plotting “imminent” and “massive” attacks on the Ramstein Air Base, a major American and NATO military hub, and Frankfurt’s busy international airport, German authorities said today.

A German federal prosecutor, Monika Harms, said the three had trained at terror camps in Pakistan and procured some 1,500 pounds of hydrogen peroxide for making explosives. And a top legislator said the group could have struck “in a few days,” noting a “sensitive period” that includes the anniversary of the September 11, 2001, attacks.

Officials said the hydrogen peroxide, stored in a hideout, could have been mixed with other additives to produce a bomb with the explosive power of 1,200 pounds of TNT.

“This would have enabled them to make bombs with more explosive power than the ones used in the London and Madrid (transit) bombings,” the head of Germany’s Federal Crime Office, Joerg Ziercke, said at a joint press conference with Harms.

The three suspects — two Germans and a Turk — first came to the attention of authorities because they had been observing an American military facility at the end of 2006, officials said. All three had undergone training at camps in Pakistan run by the Islamic Jihad Union, and had formed a German cell of the group.

The Islamic Jihad Union was described as a Sunni Muslim group based in Central Asia that was an offshoot of the Islamic Movement of Uzbekistan, an extremist group with origins in that country.

“There was an imminent threat,” German Defense Minister Franz Josef Jung told ARD broadcaster.

The three suspects were brought before judges in a closed proceeding at Germany’s Federal Court of Justice in Karlsruhe after being flown in by helicopter, court officials said.

Prosecutors in Karlsruhe said the arrests took place yesterday afternoon, and that police had also carried out searches across the country. The German reports came a day after Denmark authorities said they had thwarted a bomb plot when authorities rounded up eight alleged Islamic militants believed to have links to Al Qaeda.

Wolfgang Bosbach, a top legislator for German Chancellor Merkel’s Christian Democrats, said that “the suspects had been under observation by security officials for a long time”

“Consequently, we know without any doubt that they were planning attacks that would have had considerable consequences,” he told N24 television, adding that the three had acquired chemicals for the plot.

Mr. Bosbach said an attack could have occurred “in a few days” and pointed out the September 11, 2001, anniversary, as well as parliamentary deliberations in the next few weeks over whether to extend troop mandates in Afghanistan.

“We are in a highly sensitive period,” he said.

Ramstein is one of the best-known American Air Force bases worldwide because it serves as a major conduit for American troops moving in and out of Europe, Asia and the Middle East. It is a key transit point for injured troops from Iraq and Afghanistan who are flown there to be taken to nearby Landstuhl.

Besides American personnel, British, French, and other international forces are also located there.

Frankfurt International Airport is Europe’s third-busiest airport, handling hundreds of in- and outbound flights to and from the Americas, Asia, Africa, the Middle East and Europe. In July, a record 5.2 million passengers arrived or departed from the airport. In 2005, more than 52.2 million passengers came through the airport, which is also a major cargo hub.

German and American officials have warned of the possibility of a terrorist attack, and security measures have been increased. Navy Captain Jeff Gradeck, spokesman for the American military’s European Command in Stuttgart, said German authorities had contacted them concerning the alleged plot, but had no further information.

“We extend our gratitude to Germany for their efforts in protecting us,” Captain Gradeck said.

Germany, which did not send troops to Iraq, has largely been spared terrorist attacks such as the train and subway bombings in Madrid and London – although its involvement in the attempt to stabilize Afghanistan against Islamic insurgents has led to fears it might be targeted.

In July 2006, two bombs were placed on commuter trains but did not explode. Officials said that attempt was partly motivated by anger over cartoons portraying the Prophet Muhammad in a Danish newspaper. Several suspects are on trial in Lebanon, and a Lebanese man has been charged in Germany.

Yesterday’s arrests in Denmark sent jitters through a country that was the focus of Muslim anger and deadly protests over the cartoons. Jakob Scharf, head of the PET intelligence service, said that the eight suspects arrested were “militant Islamists with connections to leading Al Qaeda persons.”

Separately in Denmark, four Muslim men went on trial today in an unrelated case on charges of making bombs for a planned terror attack, a year after they were arrested. The defendants, who cannot be named under a court order, are accused of purchasing chemicals and equipment to produce explosives. All pleaded innocent.

The European Union’s top justice official said today that the threat of a terror attack remained high in the 27-nation bloc. EU Justice and Home Affairs Commissioner Franco Frattini said the EU executive would push ahead with plans to set up an EU-wide airline passenger data recording system despite privacy concerns.

“The threat of new terror attacks continues to be high,” Mr. Frattini said, citing Spain, Italy, Belgium, Britain, and Germany as countries where the risk has been the highest.

The German chancellor said in an interview released today that German troops would remain in Afghanistan for several more years, despite recent setbacks in the region.

“To walk away would send the wrong signal,” Ms. Merkel told N-24 television.

Magnus Norell, at the Swedish Defense Research Agency, said while Germany’s mission in Afghanistan could be a motive for a terrorist attack, a flood of other factors could also play a role.

“It could be discontent with Germany, or even western Europe as a whole. It’s really not that easy as to say that this (Afghanistan) would be the reason for it.”