Washington ? Al-Qaida is the leading terrorism threat to the United States followed by the nuclear ambitions of Iran and North Korea, the nation’s intelligence chief said Thursday in a forum that turned into a debate on government eavesdropping.

National Intelligence Director John Negroponte tried to focus on terrorist threats, but lawmakers repeatedly returned to the uproar surrounding the National Security Agency’s surveillance program.

West Virginia Sen. Jay Rockefeller, the Intelligence Committee’s senior Democrat, called the operations the largest NSA program within the United States in history. He accused the Bush administration of using the program politically while keeping the vast majority of Congress “in the dark.”

Negroponte and his top deputy, Gen. Michael Hayden, fiercely defended President Bush’s authorization allowing the NSA to eavesdrop – without first obtaining warrants – on international communications of people on U.S. soil who may be linked to al-Qaida.

“This was not about domestic surveillance,” Negroponte said in his first public words on the subject. “It was about dealing with the international terrorist threat in the most agile and effective way possible.”

Neither Negroponte nor Hayden would say publicly how many people have been monitored. Nor would they offer details on attacks that have been averted.

Hayden called the process used to determine whether someone is linked to al-Qaida “a science” – not an art – and asserted that the information that is subsequently revealed is handled lawfully.