Hamburg authorities on Wednesday defended their decision to seal off a German military hospital, saying they had a concrete U.S. tip that Islamic extremists planned an attack.

More than 100 officers in bulletproof vests and dozens of armored police vehicles blocked off streets surrounding the hospital in the northern city and searched cars after the tip from U.S. officials, the security authorities said.

The ongoing action reflected European fears of possible terror attacks around the holidays. In Italy, the government closed access to airspace over Rome to private planes until Jan. 6.

Hamburg police said early Wednesday they had turned up nothing suspicious and no one had been arrested, but the hospital in the suburb of Wandsbek (search) would remain under tight security.

German Interior Minister Otto Schily (search) criticized Hamburg authorities for acting too swiftly, saying the tips were unsubstantiated and that making the issue public hampered efforts to follow up on them.

Hamburg officials argued the information was concrete and the city's recent history with terrorists meant they had to act quickly. Hamburg was home to an Al Qaeda (search) cell that included three of the Sept. 11, 2001, homicide pilots.

"Both the federal and state branches of the German Agency for Internal Security (search) informed us in writing about the concrete risk of a terror attack," Hamburg interior minister Kirk Nockemann (search) said.

Nockemann said the tip concerned two alleged homicide attackers linked to the Ansar al-Islam (search) group who entered the country earlier this month. They allegedly had plans to target a German military facility in Hamburg with a car bomb in the first weeks of January, he said.

Some German media also accused Nockemann, whose party rode to power in Hamburg on an anti-immigration platform and beefed up security following the Sept. 11 attacks, of overplaying the threat to draw attention to his policies.

Security officials in the western state of Hessen said Wednesday that they had increased security at "particularly sensitive facilities," including the U.S. Rhein-Main Air Base in Frankfurt and another U.S. military base in nearby Wiesbaden, after receiving information from U.S. authorities of possible terror threats. He refused to elaborate.

"We have received the same information as our colleagues in Hamburg," said Peter Freier, a state interior ministry spokesman. "We are in constant contact with U.S. and German federal authorities."

Officials with the U.S. Air Force Europe have said they were not aware of any threat to their bases.

U.S. officials believe the northern Iraq-base Ansar al-Islam has links to Al Qaeda and recruits members in Europe for homicide missions in Iraq. In November, Hamburg police arrested the alleged ringleader of a group seeking recruits for a training camp run by Ansar al-Islam.

The Italian government's decision to limit access to Roman airspace came after Premier Silvio Berlusconi told an Italian newspaper terrorists planned to attack the Vatican (search) with a hijacked plane on Christmas Day. The premier later distanced himself from the comments, but his office didn't deny he said them.