Germany has increased its security measures in recent days, officials said Friday, amid intelligence reports suggesting increased threat of attacks against the country's nationals in Afghanistan — or even suicide bombings in Germany itself.

Among the reasons for concern was the arrest in Pakistan of three German Islamic radicals, believed to be part of a group of 10 people trained to carry out suicide bombings, a top law enforcement official said.

Joerg Ziercke, head of the Federal Crime Office, Germany's equivalent to the FBI, gave few details of the arrests. The suspects were young Germans who had converted to Islam and been radicalized, he said.

Ziercke cited an apparent video of a Taliban graduation ceremony for recruits — broadcast by U.S. ABC News on Thursday — that contained threats against not just the U.S., Britain and Canada but also Germany. He said that was "a new quality of threat" and said border controls had been tightened.

German officials compared the current intelligence situation to that before the Sept. 11, 2001, attacks in the United States, citing a "mosaic" of clues and previous attacks. Germany has some 3,000 troops and several reconnaissance jets as part of the international security force in Afghanistan.

German troops are stationed in the north, a relatively stable region, though three German soldiers were killed in a market bombing May 19 in Kunduz.

"There are indications that terrorist structures in the region of Afghanistan and Pakistan have gained strength and are capable of acting," Christian-Guenter Sachs, an Interior Ministry spokesman, said at a news conference. "We have information that persons from Europe, from Germany, are involved in these structures."

Sachs said the situation is reminiscent "of the early summer of 2001."

"There are increasing pieces of information, from our own services, from friendly services, indications from the Pakistan-Afghanistan region which permit conclusions that the danger level, as I have also laid out, is high for German interests," he said. "There is also a dangerous dimension, for example from suicide attackers, in the direction of Germany."

Security authorities are "at an increased state of vigilance," he said, declining to provide details of the actual measures.

German officials cited an attack Saturday on a convoy outside Kabul that included vehicles from the German Embassy, in which no one was hurt.

They also pointed to the bombing of a police academy bus in Kabul Sunday that killed at least 35 people — the same day the European Union took over a police training mission from Germany.

Three of the suicide pilots in the Sept. 11 attacks, including ringleader Mohamed Atta, lived and studied in Germany.

But Germany, which did not send troops to Iraq, has largely been spared terrorist attacks itself — although its involvement in the attempt to stabilize Afghanistan against Islamic insurgents has led to fears it may be targeted.

Interior Minister Wolfgang Schaeuble said that suicide attacks in Germany are "unfortunately ... possible," but added that recent threat information was "nothing qualitatively new."

In July 2006 two gas bombs were placed on commuter trains but did not explode in an attempt security officials said was 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.

German officials have said they have increased security around U.S. military bases in Germany, and the U.S. Embassy has advised citizens to exercise added caution.