International hackers scan New York Police Department computers at least 70,000 times a day hunting for an unauthorized entry into the system of the nation's largest police force, commissioner Raymond Kelly said Wednesday.

But all attempts have failed because of a protective system quietly constructed in the past seven years, even though hackers illegally scan NYPD computers every day, using IP addresses predominantly from China and the Netherlands, Kelly said.

"It's a threat we must continue to pay close attention to every day," said Kelly, suggesting the NYPD has become a target for hackers because of its increased international presence in the years since the Sept. 11 attack that destroyed the World Trade Center.

In an address to the Council on Foreign Relations, Kelly said the hacker attacks on NYPD computers are similar to a recently revealed breach that occurred nearly two years ago of a high-tech jet fighter program developed for the Pentagon by Lockheed Martin Corp.

"It's roughly like taking a pail of water and throwing it against the wall to see where the leaks are," Kelly said after his speech of the hackers' searches.

Kelly's speech focused on how the police department handles the threat of terrorism in the years since Sept. 11. The city has been the subject of seven major terrorist plots since then, including a plan to bring down the Brooklyn Bridge, and to release cyanide gas in the subway systems.

"As the memory of Sept. 11th fades, we in the police department must never forget that New York remains the world's most enduring terrorist target," Kelly said.

Kelly touted several initiatives aimed to protect the city from terrorism, and highlighted the department's efforts to work with other countries to better understand and prepare the city. He cited a program that sends detectives to cities like London, Madrid, Paris, Singapore and Tel Aviv, where they partner with local police agencies and report back on the methods used to respond to terrorist attacks.

The department also has recently opened a surveillance center to protect lower Manhattan from terrorist threats, the first phase of a $100 million project, Kelly said. The project will rely largely on 3,000 closed-circuit security cameras covering roughly 1.7 square miles in and around the financial district. Hundreds of cameras are already in place, with more coming in the next few years.

Dozens of private companies have also given the department access to live surveillance camera feeds as well, Kelly said.

Some of the programs have drawn fire from civil rights groups that claim the department is taking a step too far in efforts to protect the city.

But Kelly said the department's efforts are necessary to combat terrorism and be prepared should another attack occur.

The police department has more than 30,000 officers, the largest in the country.