Five years after the attack on the World Trade Center, Islamic extremists — including members of Hezbollah — still view the city as a prime target for another terrorist strike, police say.

The threat "is a permanent condition, and in all likelihood will worsen," the New York Police Department's top counterterrorism official, Richard Falkenrath, told an audience of private security executives Thursday.

"It's a very sobering conclusion and I don't reach it at all happily, but I do think it is true," he said.

The NYPD summoned the executives to offer the terrorism threat assessment and to outline security measures for the upcoming United Nations General Assembly.

Police Commissioner Raymond Kelly said there were no specific threats involving the annual U.N. event opening Sept. 12. But guarding the concentration of world leaders "always is a large undertaking for us," he said.

Aside from putting hundreds of additional officers on patrol, authorities plan to create a frozen zone around the United Nations using checkpoints fortified by hydraulic metal barriers. All vehicles entering the area will be stopped and checked for explosives.

The briefing at Police Headquarters in lower Manhattan was part of a program designed to encourage more vigilance by private security at large hotels, Wall Street firms and other companies.

While al-Qaida operatives and their homegrown imitators remain the greatest concern, Hezbollah should not be ignored, police officials said. They emphasized the point by showing footage of a Hezbollah rally in Lebanon where a massive crowd chanted, "Death to America."

Historically, the Lebanon-based Hezbollah, which is backed by Iran and is called a terrorist group by the United States, has been most active in the Middle East, but "we're worried that may be changing," Falkenrath said.

The city has pockets of Hezbollah loyalists who could be incited to violence, the NYPD intelligence analysts said.

They said they suspect that Iranian spies working on the group's behalf had already done reconnaissance on landmarks, large synagogues and other potential targets in Manhattan and elsewhere.

In 2004, the United States expelled two guards at Iran's U.N. mission for photographing "sensitive" sites around the city. Iranian officials denied any wrongdoing, saying that the pair took pictures of typical tourist attractions.